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Sunflowers bud-inoculated with Plasmopara halstedii became systemically infected with downy mildew and produced infected seeds at 15 to 30 °C. Oospores were observed in seeds of inoculated and also naturally infected plants from the field. Infected seeds gave rise to symptomless plants. Infected seeds proved effective as inoculum, inducing infection in 14 to 89% of the plants inoculated at 20 °C. Most of the infections (80%) were symptomless (latent).Systemic infection occurred, but latent infection was more frequent, in plants grown in soil containing debris of mildewed plants. Latent infection also occurred in uninoculated plants through belowground contact with systemically infected plants. Seed produced by plants with latent infection may carry the pathogen, and may give rise to more plants with latent infection, accounting for widespread dissemination of the disease before plants with typical symptoms are observed.

Of the different dates of seeding tried, the last week of June proved to be the best time to avoid the incidence of downy mildew of Sorghum. The reduction in the yield of grain and fodder is directly proportional to the severity of the disease. The disease is suppressed considerably by an available soil moisture (ASM) of 76 to 79 per cent but favoured by an ASM of 44 to 47 per cent during the period of 16 days after seeding. Soil temperature of 26.6°C during this period appears to be the optimum for the disease expression. The pH of the soil is not altered by the incidence of the disease or by the growth of the crop.

Fixation of 14CO2 in the dark by cucumber leaf infected with the cucumber downy mildew fungus, Pseudoperonospora cubensis (Berk. et Curt.) Rostow., was examined, and compared with that in the natural sun light condition.The radioactivity (cpm/10mg dry weight) of 14C fixed in lesions immediately after the exposure to 14CO2 in the dark was 601-813. These figures were corresponded to 2.7 to 3.7 times as high as that in the healthy part of the same leaves. The radioactivity of 14C in lesions decreased to 293-344 1 day after the treatment with 14CO2. However, the radioactivity of 14C fixed in lesions in the dark was extremely little as compared with that of 14C fixed in lesions through the photosynthetic pathway in the natural condition (under the sun light about 50, 000Lux). That is, the radioactivity (cpm/10mg dry weight) of 14C fixed in the dark was 93, while that of in the natural condition was 38, 216.The 14C fixed in lesions both under the natural and dark conditions was incorporated into conidia on infected leaves. There existed, however, large differences between the natural and dark conditions in the amount of 14C incorporated into the conidia. They were 439, 139 in the natural condition and 1, 464 in the dark, respectively.

Scientific development of plant pathology in Japan began with the introduction of European and American civilization in the early years of the Meiji era, although some accounts concerning plant diseases have been found in earlier records. These records show that the ancient people recognized that plant diseases were injuring their crops, but because they had no conception of the nature and cause of these maladies, they superstitiously observed mysteriously deformed diseased plants with surprise and terror. According to Shirai (90) there is an account in the historic book Zoku-Nihon-Ki that in January 713 A.D. deformed rice plants were presented to the Emperor. These were thought to be a transformation of grasses into rice (27, 72). According to Shirai (90), these could have been rice ears infected by downy mildew (Phytophthora (Sclerophthora) macrospora). Ganoderma lucidum is a comparatively common decay fungus in Japan, attack­ ing stems and roots of coniferous and deciduous trees (21). From very early times development of the fruit body of this fungus was considered to be a sign of happy events. Many descriptions and much information on this are found in the classics; accounts in Nihon-Shoki show that in 678 A.D. the fruit body of this fungus was presented to the Emperor and that in 726 A.D. this fungus developed fruit bodies in the Imperial Palace (27). Many ancient classics indicate that blast disease of rice plants was very serious and, together with unfavorable weather conditions, caused much damage to rice crops. People then suffered severe famine (27, 72). The first record of blast disease of rice in Japan appears in a book, Koka Shunju (45, 101), written by M. Tsuchiya and published in 1707. He emphasized the relation between environmental condi­ tions and occurrence of the disease. Miyanaga (65), Horii (35), Kojima (57), and Konishi (58) also referred to rice blast disease in their books. Konishi (58) empha­ sized especially the relationship of cultural conditions of rice plants (e.g. nitrogenous fertilizers, deep plowing of soil, density of seeding) to the occurrence of blast disease. According to Hino (27), accounts concerning the following plant diseases or fungi

Since June 1973 Pseudoperonospora cubensis (Berk & Curt) Rost., which causes downy mildew in cucumber, occurs in the Netherlands. The resistance against this disease appears to be based on one recessive gene in linkage with the dominant gene D for dull green fruit skin colour. It is demonstrated that this recessive gene is also linked with one of the genes for resistance to powdery mildew present in the variety Ashley.

Lignin is formed in the cell wall of the downy mildew-infected Japanese radish root and peroxidase activity increases in the neibouring tissues. Attempts were made to study the functional role of this increased peroxidase activity in lignification in diseased tissues. The crude peroxidase prepared from the healthy tissue with ammonium sulfate precipitation and isoelectric point precipitation was fractionated into 11 fractions by cellulose column chromatography. The activity of the crude enzyme preparation increased gradually in response to the infection, reached a maximum 9 days after inoculation and then decreased. Elution profiles of peroxidase extracted from the root 9 days after inoculation showed that activities in acidic and neutral fractions apparently decreased while those of basic fractions increased. Polyacrylamide gel electrofocusing of the crude peroxidase preparation showed a decrease in activity of acidic isoperoxidases and an increase in activity of a part of basic and neutral isoperoxidases in the diseased tissue.

Distribution of 14C photosynthetic products in cucmber leaf infected with the cucumber downy mildew fungus, Pseudoperonospora cubensis (Berk. et Curt.) Rostow., was studied for elucidating the relationship between the fungus and the photosynthetic products in infected cucumber leaves.When the plant was exposed to 14CO2 1 day before inoculation, 14C photosynthetic products 5 and 7 days after inoculation were not accumulated in lesions. The infected plant with no visible symptoms was exposed to 14CO2 2 days after inoculation. 14C photosynthetic products were accumulated 6, 8 and 14 days after inoculation in freckle areas which seemed to appear at an early stage of the symptom development. When the infected plant was exposed to 14CO2 8 days after inoculation, the amount of radioactive carbon in lesions was estimated to be less than that in the healthy part of the same leaves immediately after the exposure to 14CO2. During the subsequent period from 7hr to 5 days after the treatment with 14CO2, 14C photosynthetic products were accumulated in lesions, and the radioactivity in lesions was higher than that in the healthy part of the same leaves. On the other hand, the radioactivity immediately after the exposure to 14CO2 in the healthy part of infected leaves decreased rapidly as compared with that in the leaf from healthy plants. 14C photosynthetic products were translocated within 1hr after the beginning of 14CO2 treatment from the healthy part to lesions in the same infected leaves.The amount of radioctive carbon in conidia was much more when sporulated on the day of the exposure of infected leaves to 14CO2 than that when sporulated on 1, 2 and 3 days after the exposure.It is considered from these results that 14C photosynthetic products were translocated within a very short period after the exposure to 14CO2 from the healthy part to lesions in infected leaves, and that they were incorporated rapidly into mycelia and conidia of the fungus.

Fifty-eight lettuce cultivars were tested for their reaction to three races of downy mildew in Israel and to four races in the Netherlands. It was found that in these two countries at least seven different races exist. The inheritance of resistance to each race was established.

SUMMARY The inheritance of resistance to lettuce root aphid, Pemphigus bursarius, was studied in lettuce using the Wellesbourne cultivars Avondefiance and Avoncrisp as resistant parents and Borough Wonder and Webb's Wonderful as aphid-susceptible parents. All four cultivars were crossed in all possible combinations including reciprocals and the response to root aphid of plants in the P1F1F2 and BC generations was assessed using apterae of P. bursarius from the lettuce cv. Iceberg. Resistance to attack was clearly inherited and the parents appeared to be homozygous for their resistance or susceptibility. In the F1 generation, however, in all crosses between resistant and susceptible parents, segregation into susceptible, resistant and some slightly less resistant plants occurred. This and the highly significant differences in segregation between pairs of reciprocal crosses in the F1 and other generations indicate that the inheritance of resistance to root aphid is controlled by extra-nuclear factors. Modifying genes might also be involved but there appears to be no linkage of root aphid resistance with resistance to downy mildew, for which the Wellesbourne lettuces were bred.

Summary: Two sprays of streptomycin (1000 μg./ml.) considerably reduce sporulation of the hop downy mildew fungus, Pseudoperonospora humuli, on leaves of primary systemically infected shoots. Streptomycin markedly inhibited germination of its sporangia in vitro. Toxicity was dependent on sporangial concentration, incubation temperature and the cleanliness of the suspension but was independent of sporangial age. There were no marked differences in sensitivity to streptomycin between sporangia from five different isolates of Ps. humuli. Two commercial formulations of streptomycin were as inhibitory to sporangial germination as pure streptomycin sulphate. Sublethal concentrations of streptomycin interfered with cleavage of sporangial cytoplasm and increased the proportion of multiflagellate zoospores. The proportion of multiflagellate multinucleate zoospores liberated was dependent on streptomycin concentration. Germination of multiflagellate zoospores was observed. Streptomycin was very inhibitory to zoospore germination but had little effect on zoospore motility and germ-tube growth at concentrations which caused a high incidence of cyst lysis. Streptomycin was as toxic to sporangia of Phytophthora infestans as to those of Ps. humuli, whereas sporangia of Plasmopara viticola and Peronospora parasitica were less sensitive to the antibiotic.

Summary 1. Lettuce seedlings, grown from both ordinary commercial seed and seed from mother plants known to be infected with mildew, almost invariably failed to show symptoms of the disease under conditions favourable to mildew development. There was thus negligible evidence of the seed-borne transmission of mildew. 2. Oospores were not found by the microscopical examination of debris from infected plants, and seedlings grown in soil containing a high proportion of such debris for a period considered to be adequate, under favourable conditions for the development of mildew, did not become infected. The transmission of the disease by contaminated soil is therefore unlikely. 3. Attempts to transmit mildew from composite weeds to lettuce were successful only with some wild species of Lactuca . These species are probably too rare to be important in the spread of the disease. 4. The survival of mildewed lettuce was lower, and their date of maturity later, than that of healthy plants. This, together with the disfigurement caused by the disease, gives it its commercial importance. 5. A number of commercial lettuce varieties commonly grown in England showed varietal differences in their susceptibility to the pathogen but these differences were not sufficiently marked to make the exclusive cultivation of the more resistant varieties worth while as a control measure. 6. 2:2:40 Bordeaux mixture, Excess-Lime Bordeaux (2:10:40), an orthophosphate copper spray, and Cuprocide 54 (1 lb. to 50 gal.) all gave adequate protection from mildew disease but sprays containing no copper such as lime sulphur and pentachloronitrobenzene did not. All the copper- containing sprays are liable to produce phytocidal damage, particularly in cold weather. 7. Fungicidal vapours such as benzol and paradichlorbenzene were troublesome to use and gave a poor control of lettuce mildew. 8. Bremia conidia under favourable temperature conditions germinated readily in saturated lime solution and were also found to germinate more readily in slightly alkaline than in slightly acid solution.

The present report dwelt upon the observations on downy mildew of wheat and its causal fungus, Phytophthora macrospora.As for the causal fungus, only the oospore had been known until TASUGI discovered for the first time its conidial stage on rice in 1927. He reported in detail the morphological character of the fungus in 1930, and he stated that the causal organism should be transfered to Nozemia from Sclerospora.The author happened to find the conidial stage of the fungus on wheat-plants badly affected by the downy mildew after a flood at Ikeda, Prov. Tokachi, Hokkaido in 1937. The conidia produced on wheat leaves in the fields were 57.5-97.5μ in length and 30.0-65.0μ in width; average 74.6×46.8μ. When these leaves were soaked in water, the spores elongated to 65.0-112.5×32.5-55.0μ; average 87.37×43.75μ. The latter measurements were correspondent to those reported by TASUGI about the conidia discovered on affected rice plants. Oogonia formed in the tissues of affected wheat were 55.0-100.0×50.0-95.0μ; mostly 62.5-75.0×56.5-72.5μ; average 69.12×65.13μ. Antheridia paragynous, 22.5-41.25×12.5-22.5μ; average 27.75×15.15μ. Oospores, . 42.5-72.5×42.5-70.0μ; average 60.25×56.67μ. Zoospores 12.5-16.25×10.0-12.5μ; average 14.17×10.8μ. The conidia were produced abundantly at 11°-24°C.According to the morphological characteristics of the conidia it was concluded that the causal organism undoubtedly belongs to Phytophthora. Having compared the conidia and oospores formed on wheat and rice with those on other affected cereal crops and wild grasses, the author came to the conclusion that Sclerospora macrospora SACC., S. Kriegeriana P. MAGN, , S. Oryzae BRIZI are one and the same species.Hereby the author propose to change the nomenclature of the causal organism as follows:Phytophthora macrospora (SACC.) S. ITO et I. TANAKA n. comb.=Sclerospora macrospora SACC. Hedw. XXIX, 155, 1890: Syll. Fung. IX, 342, 1891-BERL. Monogr. Peronosp. 148, 1903.=Sclerospora Kriegeriana P. MAGN. Verh. Ges. Deutsch. Nat. u. Aerzte, 67; Vers. zu Lubeck, 1895, 100-SACC. et SYD. in SACC. Syll. Fung. XIV, 461, 1899.=Sclerospora Oryzae BRIZI, La Peronospora del Riss, in Nature, X, 168, 1919-TROTT. in SACC. Syll. Fung. XXIV, 65, 1926.

Agricultural practices such as irrigation and year-round cultivation of crops, render semi-arid areas much more suitable for development of downy mildews and late blight than might be supposed by weather criteria alone. These diseases can develop even under not very favorable humidity conditions, owing to a favorable microclimate and to abundant pathogen reproduction.

PEYTON, G. A., and C. C. BOWEN. (Iowa State U., Ames.) The host-parasite interface of Peronospora manshurica on Glycine max. Amer. Jour. Bot. 50(8): 787-797. Illus. 1963.-The fine structure of the vegetative intercellular hyphae, intracellular haustoria, and invaded host cells is described. Perinuclear Golgi apparatus and extensive lomasomes are characteristic of thehyphae and haustoria of this fungus. The invading haustoria do not penetrate the plasma membrane of the host. Except for a sheath near the point of penetration, there is no evidence of true host wall around the haustorium. However, a "zone of apposition," with staining properties different from those of normal host cell wall, forms around the haustorial wall between the host and parasite plasma membranes. Special modifications of the host cytoplasm in the vicinity of haustoria are described, including formation of "secretory bodies" and their apparent discharge through the host plasma membrane into the zone of apposition. This phenomenon, together with an apparent increase in the number of ribosomes in the host, suggests highly specific reactions of the host cytoplasm to the invading haustorium. AN UNDERSTANDING of the organization of the interface between host and parasite at the cellular level is basic to the study of biochemical interactions between the 2 organisms and to a better understanding of parasitism and pathogenesis. This is especially true of obligate parasitism where infection often exists for some time without causing a great deal of apparent harm to the host and where unique structural and biochemical relationships must occur. The anatomical relationships between host and parasite have been investigated in a number of fungus plant diseases with the light microscope. Investigators were hampered by the small size of the fungal cells involved and by the limited resolving power of the light microscope. Despite these limitations, many astute observations were made, and many details of host-parasite relationships have been described. The work of Rice (1927) and Fraymouth (1956) is representative. The application of the greater resolving power of the electron microscope to the investigation of plant diseases caused by fungi has been limited to a brief study of Uromyces caladii by Moore and McAlear (1961b) and a somewhat more extensive investigation of Puccinia graminis by Ehrlich and Ehrlich (1963). A careful study of the fine structure of a host-parasite relationship involving a Phycomycete would add significantly to the understanding of parasitism and pathogenesis. 1 Received for publication January 20, 1963. This paper is based on a portion of a thesis submitted by the senior author to the Graduate Faculty of Iowa State University in partial fulfillment of the requirements of the degree of Master of Science. This investigation was supported in part by research grant C-3982 from the National Institutes of Health, United States Public Health Service. The authors appreciate the advice and assistance given by Dr. John M. Dunleavy. 2 Present address: Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan. Peronospora manshurica (Naom.) Syd. ex Gaum., causal agent of downy mildew of soybeans, is an obligate parasite and a typical member of the large group of downy mildew fungi. This study is limited to the vegetative aspects of infection; the many aspects of parasite reproduction, both sexual and asexual, will be left for future work. MATERIALS AND METHODs-Production of diseased and control material Throughout this study, soybeans, Glycine max (L.) Merr. cultivar 'Illini,' were used for both infected and control material. Race 8 of P. manshurica, to which 'Illini' soybeans are highly susceptible (Grabe and Dunleavy, 1959) was used throughout the investigation. Potted seedlings were infected by conidia applied in suspension. Subsequently, infected plants and healthy controls of the same age were placed in a constant-temperature growth chamber at 21 C under 450-750 ft-c of illumination, with 18 hr light and 6 hr darkness. Leaves were removed for study from infected and control plants at intervals ranging from 7 to 60 days after infection. Specimen preparation for electron microscopyA circular punch was fashioned from a no. 18 Yale BD hypodermic needle. The needle was attached to a syringe containing a small amount of water and used to punch out uniform circular pieces of leaf tissue 0.8 mm in diameter. The pieces of tissue were then expelled into the fixative and promptly evacuated to insure penetration of the fixative. Fixation was accomplished by one of the following procedures: (1) 1% osmic acid in 0.2 M potassium phosphate buffer (Ledbetter, M. C., Cambridge, Massachusetts, private communication, 1961) at pH 7 for 30 min at room temperature; (2) 4% unbuffered potassium permanganate (Mollenhauer, 1959) for 10 min at room temperature; (3) and (4) 4% formaldehyde and 7.55% sucrose in

SUMMARY Sugar-beet downy mildew is most prevalent in England in the sugar-beet and mangold seed-growing area of South Lincolnshire and West Norfolk. The most widespread and severe recent outbreaks were in 1957, and in 1965 when 6412 acres were reported with more than 10% infected plants. The fungus usually overwinters in sugar-beet and mangold seed crops, and in England other ways of overwintering are seldom important. Steckling beds are infected in the autumn, and the disease may increase rapidly in the seed crop in early spring. Summer-sown stecklings get more downy mildew than stecklings sown in spring under a cereal cover crop, and direct-drilled seed crops get more downy mildew than transplanted crops.

Downy mildew of onion appeared initially in the semi-arid British Columbia Interior about 50 years after onion crops had been first grown commercially. The year of its appearance, 1942, was abnormally wet, and this condition suggested that the weather had previously been unfavourable for its establishment. However, the disease has since persisted and has caused damage in most seasons. The pathogen was probably present on infected living plants introduced shortly before the appearance of the disease.Systemic overwintering of the pathogen in diseased bulbs occurs only occasionally. Heat treatment of seed mother bulbs was found to injure them sufficiently to seriously reduce the number and height of seed stalks.Zineb provided best control in fall-planted, spring-planted, and seed crop onions. Maneb and ferbam were less effective. Captan and NP–1282 were ineffective. Control was found to be effective only if a spray was applied at first indication of mildew in a district.

SUMMARY When various fungicides were sprayed on detached cabbage cotyledons in the laboratory and on young plants in the glasshouse, propineb and mancozeb at 0·2 % a.i. consistently gave good protection against subsequent infection by spores of Alternaria brassicicola. Copper oxychloride, zineb, thiram and maneb showed promise in the laboratory, but were less successful in the glasshouse. Two fentin compounds were active at low concentrations against the fungus but were phytotoxic. Increased pathogenicity of A. brassicicola in the presence of cabbage pollen and of downy mildew infection in the laboratory is reported.

Downy mildew (Peronospora matthiolae (Roumeguerre) Gaumann) of stock, which was first found in New Zealand in 1958, became a serious and widespread disease within a year. Life history studies of the fungus indicated that it produced only asexual conidia, and that it was not systemic in the plant. Amongst the Cruciferae the fungus infected only species of M. atthiola, both M. incana and M. bicornis being susceptible. All varieties of stock tested were susceptible, but there was evidence that intensity of sporulation varied between varieties. Under optimum conditions the incubation period of the fungus was 5–6 days. It required high humidities for sporulation, and the optimum temperature for infection and sporulation was 15.5°–21°c. Streptospray (500 p.p.m. + glycerol, 1 per cent) protected plants from infection, and also acted as an eradicant. Sprays containing ferbam, cuprous oxide emulsion, Phaltan, dichlone, U.C.P. 21, Trioneb, and zineb gave protection but were not eradicant. Best control of n...

SUMMARY In glasshouse and field experiments, Avonearly red beet at the seedling and young plant stage was more resistant to downy mildew than several other commercial cultivars. Similar resistance was observed in Avonearly stecklings overwintered in frames or in a cool store (maximum temperature 4·4°C)for subsequent seed production. This resulted in better survival, both during the winter and the following summer, than was shown by the mildew-susceptible cv. Detroit Improved Globe.

When zoospores of Pseudoperonospora humuli in water were inoculated in the light on to abaxial surfaces of hop leaf disks, which had been maintained in light, they began to respond to stomata within 2 min. After 4 min a proportion significantly greater than random had settled on stomata. Typically, only one zoospore was associated with each stoma. In darkness, zoospores remained motile for longer periods and showed no preference for stomata, finally settling on the leaf surface at random. Zoospores of Plasmopara viticola in light settled on stomata of grapevine leaf disks in groups of up to 10 but, in darkness, on comparatively few stomata in groups of up to 28. Two processes are involved in the typical response of zoospores of P. humuli to stomata in the light, a shortening of their motile period on the leaf surface and their selection of stomata as sites of settling. Analysis of zoospore behaviour on hop leaf disks subjected to various light and chemical treatments and on plastic replicas of leaves with open or closed stomata showed that two stimuli, one chemical, involving photosynthesis, and the other physical, from open stomata, contribute independently to the complete response. Whereas both chemical and physical stimuli encouraged zoospores to select stomata, only the chemical stimulus caused their motile period to be shortened. Various aspects of the zoospore response to stomata of leaf disks and replicas are illustrated in scanning electron micrographs.

SUMMARY Infection of hop rootstocks by Pseudoperonospora humuli has been shown to occur as a result of inoculation of the tips of shoots 15 cm. or less in length. No rootstock infection developed when shoots were more than 17.5 cm. long at the time of inoculation. Infection via the base of shoots can occur at any time during the growing season and is thought to be the main source of rootstock disease under natural conditions. The importance of direct infection of hop rootstocks by zoospores of P. humuli, which occurs readily under laboratory conditions, is not known. Application of Bordeaux (6.5% Cu) and captan (15% a.i.) dusts to the crowns and shoot bases of field plants retarded development of rootstock disease and shoot infection (basal spikes) in originally disease-free plantings of Golding hops.

Features of attack on hops by the two fungus diseases, Verticillium wilt and downy mildew, and the two virus diseases, nettlehead and mosaic, are described. Methods of control are indicated, and the importance is shown of tolerant varieties. The use of such varieties against wilt is becoming a matter of great urgency, and breeding for resistance to downy mildew is being carried out. The merits and dangers of symptomless carriers of mosaic are noted. Nettlehead is probably the disease causing greatest loss to the hop-growing industry.

The downy mildew (peronosporosis) is the most dangerous and harmful cucumber disease in the Far-Eastern region. Its harmfulness is the highest during the period of flowering – fruiting, because it may completely ruin the cucumber plants both in open and protected ground. Use of cucumber sorts with effective genes that are resistant to the disease is the most efficient way to fight cucumber peronosporosis. In this connection the publication adduces the facts of study the sorts and hybrids of different ecological and geographical origin from the genetic resources collection of the N.I. Vavilov All-Russian Institute of Plant Genetic Resources (VIR) in comparison with three zoned sorts of DV NIISH selection – Mig, Khabar, Amurchonok. The aim of researches – to identify the sort samples cucumber according to the main economically valuable traits and also to the degree resistance of genetypes to the main disease of the culture in the region – downy mildew (peronosporosis), for allow to select the source material for the creation of sorts and hybrids, highly productive and resistant to biotic and abiotic stress environmental factors in the monsoon climate conditions of Khabarovsk region. The perspective genetypes having both the resistance to the diseases and productivity are not revealed as a result of the evaluation of the collection of cucumber samples according to their susceptibility by fungus diseases and productivity. Slavyanskiy (Belarus) is more resistant to peronosporosis, especially in 2016 year conditions, although it was inferior to the damage degree of disease to the zoned sorts – Mig, Khabar, Amurchonok. This perspective sample is planned to be used as a source of donors of the main economically valuable traits in a further selective work.

SUMMARYIn tests with seedlings in the glasshouse, significant differences in susceptibility to downy mildew, caused by Peronospora farinosa f. sp. betae, were observed between individual stocks of sugar beet. The resistance of a particular stock in glasshouse tests was not always a reliable guide to its resistance in the field. There were also serious discrepancies between the results of tests with seedlings and those with mature plants in the field, suggesting that the resistance of certain stocks, relative to one another, may vary with increasing age of the host plant. This contention was supported by evidence from a field experiment in which the susceptibility of several sugar-beet stocks was compared throughout the growing season. It also appears that changes in environmental conditions affected resistance to downy mildew more in some stocks than in others.Progenies of selected plants that had not shown severe symptoms of downy mildew, after exposure to natural inoculation with P. farinosafor more than 3 months in the field, were much more resistant than the parent stocks. This showed that a single generation of selection under suitable conditions can significantly improve the resistance of sugar-beet breeding stocks to downy mildew.An inbred sugar-beet line of American origin showed exceptional resistance to downy mildew in experiments under glass and in the field. Inoculated plants of this line usually showed no symptoms of the disease and P. farinosa did not produce conidia on them. These results show that sources of extreme resistance are present in cultivated sugar beet and suggest that an excellent control of downy mildew in Britain may eventually be achieved through the use of resistant varieties.

In experiments using both attached hop leaves and leaf disks floating on water, a general correlation was found between the number of stomata per unit area open at the time of inoculation with Pseudoperonospora humuli and the amount of subsequent infection, assessed as area infected and degree of sporulation after 7 days. Whether stomatal opening was varied by light or darkness, chemicals or different soil moisture regimes, a similar result was consistently obtained, irrespective of leaf age or cultivar. Microscopic examination of the leaf surface after inoculation in the light with a large number of open stomata, revealed that the majority of zoospores settled and encysted singly on stomata which were then penetrated by germ tubes. In contrast, in darkness, when stomata were mostly closed, zoospores settled between stomata and few germ tubes encountered and penetrated stomata. This differential response to stomata occurred on hop leaves of various ages and cultivars and on leaves of certain non-hosts. A statistical test, based on sequential analysis, allowed zoospore-stoma associations to be evaluated with the low zoospore numbers used. In reciprocal inoculations of hop and vine leaves zoospore behaviour in Pseudoperonospora humuli was compared with that in Plasmopara viticola. Marked differences in the number of zoospores settling on single stomata occurred which appeared to be due to both stomatal and zoospore factors. The results are discussed in relation to downy mildew in the field, resistance to the disease and the possible mechanism of the zoospore response.

SUMMARY In laboratory tests on young lettuce plants, the protection provided by sprays of zineb wettable powder against artificial inoculations with Bremia lactucae was less than that obtained with tank-mix zineb, and with several other dithiocarbamates and related fungicides. Experiments on autumn lettuce in the field also showed that tank-mix zineb was more effective than the wettable powder. On winter lettuce in frames, however, zineb wettable powder gave control similar to or better than that given by any of the other materials tested. Infection in most of these outdoor experiments was not heavy. None of the fungicides gave very high control or markedly increased the number of hearted or marketable plants. When zineb wettable powder was sprayed on autumn lettuce at the rate of 2 lb./acre it gave similar disease reductions whether applied in low, medium or high volumes of water.

SUMMARY: Successful infection of hop tissue culture with Pseudoperonospora humuli was achieved. Infection was obtained from hyphae emerging from surfacesterilized systemically infected stems and petioles. Externally, infection was characterized by sterile aerial hyphae and sporangiophores, and internally by intercellular mycelium and intracellular haustoria. Although a considerable growth of mycelium into the agar medium was observed, it appeared to be dependent on the presence of host tissue. Attempts to establish axenic cultures of P. humuli were unsuccessful.

Conidia ofSclerospora sorghi, obtained from either systemically-infected or local-lesion-infected leaves of sorghum (cv. Vidan), were capable of inducing typical downy mildew systemic infection, including oospore formation, in sorghum and corn hybrids. Very young inoculated seedlings displayed chlorotic systemic symptoms already on the first leaf, and often died at fourth-leaf stage. Systemic infection was induced by conidia on sorghum 1–14 days old at inoculation. Incidence of infection was much higher and symptoms less delayed when the shoot rather than coleorhizas of young sorghum and corn seedlings were inoculated; in two-week-old sorghum with three leaves, inoculation of the coleoptile or of the base of the second and third blades resulted in systemic infection; with coleoptile inoculation partial leaf chlorosis was delayed until the fourth-or fifth-leaf stage, showing that penetration without symptoms had occurred as far as the meristematic tissues of young leaves still within the leaf tube. Conidial inoculation of young sorghum tillers sprouting after cutting down healthy mother shoots resulted in systemic infection. Conidial inoculum is deemed to be the probable major means for systemic infection of corn and sorghum sown in fields in which oospores are not present; inoculation of new tillers of forage sorghum by conidia from infected plants in a neighboring field can explain the rise in numbers of plants systemically stricken. Two sweet corn hybrids — one considered resistant in the field, the other very susceptible — proved equally susceptible when inoculated with conidia at 5 days of age.

SUMMARY The germination of conidia of Peronospora farinosa f. sp. betae, collected from sugar beet and suspended in deionized water, was inhibited by dilution with 10% solutions of glycerol, glucose or sucrose and with sap from sugar-beet leaves. Germination was stimulated by diluting with deionized water but not with tap water or biological saline. Substances that diffused from excised buds of sugar-beet plants into deionized water also stimulated germination of conidia but diffusates from leaves did not. This may partly explain why buds are more susceptible to downy mildew than leaves in sugar beet. Germination of conidia was apparently stimulated more by diffusates from buds of seedlings than by those from buds of older plants; this may help to explain why sugar-beet seedlings are more susceptible to downy mildew than older plants. Diffusates from plants of four sugar-beet stocks, that differed from each other in susceptibility to downy mildew, had very similar effects on germination of P. farinosa conidia. Stimulation of spore germination on the surfaces of buds and leaves did not seem, therefore, to be an important factor in determining resistance or susceptibility to downy mildew in these stocks.

Factors affecting the sporulability (capacity for sporulation) of cucumber downy mildew fungus, Pseudoperonospora cubensis (Berk. et Curt.) Rostow., in leaf lesions were investigated. The sporulability was evaluated on the basis of the number of conidia formed on lesion of 1cm2 after being kept for 17hr in a moist chamber under darkness at 20°C.1. Relationship between the age of lesion and sporulability is shown in Fig. 1. The sporulability attained maximun 7 days after inoculation.2. The sporulability was affected by the weather condition of the day when infected leaf specimens were taken from the plant. Conidia were more heavily formed on leaf specimens taken on a fine day than those taken on a cloudy day (Table 1).3. To know the influence of sunlight on the sporulability, a half part of diseased leaf was covered with a sheet of black paper and the other half was left uncovered and exposed to direct sunlight from sunrise to 5p.m., being about 1hr before sunset. The sporulation in the lesion on the uncovered half, which was exposed to sunlight, was always greater than that in the lesion on the covered half, as shown in Table 2. The ratio of the number of conidia formed on the covered half to the uncovered half was 40-64: 100 in the first, 54-60: 100 in the second, and 30-71: 100 in the third experiment.4. The sporulability in lesions either with or without healthy part was compared by using lesions 6 days after inoculation. Conidia were more abundantly formed on lesions with some healthy part attached than on lesions without healthy part.It was considered from these results that the sporulability of cucumber downy mildew fungus closely depends on the photosynthesis of host cucumber leaves.

SUMMARY The experiments described in this paper show that Pseudoperonospora humuli can overwinter in infected hop rootstocks in the form of mycelium and that primary infected shoots (basal spikes) are produced from dormant buds infected by such mycelium. Crop yield from plants with infected rootstocks averaged 27.7% less than that from healthy plants.

Preliminary tests made in 1929 show that cucumber plants dusted for control of cucumber beetles and downy mildew average smaller (yielded less in case of downy mildew) than those treated while dry. The dusts include: Hydrated Lime, Gypsum, these two with Calcium Arsenate, Sodium Fluosilicate, Calcium Fluosilicate, and “Copper-Lime.”

SUMMARY Applications of streptomycin sulphate or protectant fungicides reduced numbers of secondary infections in severely infected hop plantings. When primary infected shoots treated with streptomycin were left on plants for 3 weeks no significant increase in secondary disease resulted. Streptomycin, which was absorbed rapidly by hop plants, could not be detected in expressed shoot sap by early June and had no effect on crop yield or quality.

SUMMARY Experiments are described in which successive groups of healthy, susceptible, potted hop-plants were exposed, each 24 h, to the weather and to sources of Pseudoperonospora kumuli inoculum in an unsprayed hop garden for 48 h periods during May and June in 1969, 1970 and 1971. The infection which arose after standardized incubation in isolation was measured and then related, both by inspection and by multiple regression analysis, to the conditions during exposure. Severe infection could be associated with certain sequences of events in which rain contributed substantially to plant wetness; relatively light or no infection developed when dew provided the wetness of a period. Infection was markedly correlated with the occurrence of infection periods, as defined from earlier growth-room studies, especially when dew-wetness was omitted. The regression analysis showed that, in all years, infection was highly correlated with variables reflecting wet conditions. It was inconsistently correlated with vapour pressure deficit (VPD) and with sunshine, not at all with temperature, and with airborne spore concentration only in the combined years. The spore catches of a funnel trap, used in 1971, which depended upon rain varied closely with infection. R2 values for multiple regression equations indicated that, in the years separately and together, well over 70% of the variation in infection could be explained by the environmental variables and, in 1971, almost 90%. Variables (temperature, VPD, airborne spores) which singly were poorly correlated with infection often significantly improved regression equations which were based on variables expressing wetness. When prediction of each year's infection levels was attempted using selected regression equations calculated from data of the other 2 years there was, in general, very good agreement between measured and estimated values. The best predictions were given by an equation utilizing rain-wetness duration, rainfall amount and airborne spore concentration while an equation based solely on relative humidity and rainfall was only slightly inferior. The results are discussed in relation to the possible value of the regression models for short-term forecasting of hop downy mildew.