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Cavendish banana cv. Formosana is a high yielding commercial cultivar resistant to race 4 of Fusariumoxysporum f. sp. cubense. Mass micropropagation of this cultivar has a serious problem of high mortality due to lethal browning of plantlets. The mineral contents in leaves and corms of diseased and healthy plantlets were similar. Amendment of culture medium with anion exchange resins, cation exchange resins, polyvinylpyrrolidone or activated charcoal did not reduce the disease incidence. However, addition of ascorbic acid to the surface of culture medium not only prevented the development of lethal browning but also greatly increased the number of plantlets produced. Even at 0.005% ascorbic acid was able to reduce the disease incidence by more than 60% and caused over 8-fold increase in number of plantlets produced. When cultures raised from 12 different Formosana corms were tested, ascorbic acid was able to reduce disease incidence by an average of 83%, and increase the number of plantlets in each test. When diseased plantlets were transferred to culture medium with ascorbic acid, all of them recovered, and resumed normal growth and multiplication, while all control plantlets on culture medium without ascorbic acid died after one month.

During a recent survey of the inoculum source of twig dieback of tea caused by Macrophoma theicola, symptoms of canker were observed on branches of a number of tea cultivars. Most cankered areas contained minute black pycnidia of M. theicola, which ooze white conidial masses in strands when incubated under moist conditions. Macrophoma theicola was also consistently isolated from cankered branches. When healthy branches of tea were inoculated with conidia of M. theicola obtained from diseased branches, cankers similar to those occurring in nature developed in the inoculated areas, and M. theicola was re-isolated from all experimentally induced cankers. Isolates derived from conidia of M. theicola also caused dieback on healthy tea twigs, indicating that fruiting bodies on the cankered branches are the main inoculum source of twig dieback of tea. There was a positive correlation between the severity of twig dieback and the prevalence of branch canker among the tea cultivars surveyed. Six of the cultivars surveyed were highly resistant to M. theicola.

Colony-type stability of asexual progeny of selfed-oospore cultures of Phytophthora cinnamomi was investigated. Progeny were divided into parental type A, types B to E for those different from type A but with a distinct uniform pattern, and type M for those with the overlapping appearance of two different types. Colonies derived from three successive zoospore generations of isolate P630 were of the same parental A type. However, colonies from oospores of the same isolate consisted of types A, B, C, D, E, and M. Some of the oospore cultures of type A or B and all the type M oospore cultures produced zoospores containing more than one type. Oospore cultures of types C, D, or E that were tested produced zoospores with the same colony type as their respective parent. Among the five cultures randomly selected from asexual progeny produced by oospore isolates P630-21 and P630-6, four displayed continuing variation in colony type in the three to five successive zoospore generations tested. Keywords: asexual variation, continuing variation, Phytophthora cinnamomi, sexual reproduction.

The conditions for improved yields of the α hormones of Phytophthora parasitica compared with those of the original technique established by Ko are described. Both hormones al and α2 were adsorbed on the Millipore filter by exposing both sides of the filter directly to freshly inoculated 40% V-8 agar blocks for 4 days. Usage of polycarbonate membrane in the original method to maintain sterility of the filter was replaced by sterilization of the filter with ethanol. Among five organic solvents tested, 95% ethanol was the most effective in extracting both α hormones from the Millipore filter. A 1-min shaking of the hormone-loaded filter with 95% ethanol was sufficient to extract ca. 95% of hormones α1 and α2. Results on the solubility properties suggest that the α hormones of Phytophthora parasitica are lipid-like natural products and that hormone α2 is more polar than hormone α1.Key words: sex hormone, hormone production, sexual reproduction, Phytophthora parasitica.

Isolate 167F of Pythium splendens and its four selfed-oospore isolates gave rise to conidium progeny similar to their respective parents in growth rate. However, oospore progeny from these isolates displayed considerable variation in this phenotypic characteristic. These isolates also showed similarity in conidium progeny and dissimilarity in selfed-oospore progeny in sensitivity to streptomycin and chloramphenicol. Among the 117 oospore cultures obtained from isolate 167F, resistance to streptomycin and chloramphenicol was negatively correlated with the growth rates. Results from this study suggest the polygenic inheritance with alternate alleles at each locus for growth rate, sensitivity to streptomycin, and sensitivity to chloramphenicol. Key words: Pythium splendens, conidium progeny, oospore progeny, growth rate, antibiotic sensitivity.

All tested isolates of Phytophthora insolita and an unknown asexual Phytophthora species found in soil, ditch water, and diseased plant tissues in Taiwan produced ovoid, nonpapillate, nondeciduous sporangia on sporangiophores proliferating through an empty sporangium or in a nesting fashion, and formed irregular as well as spherical hyphal swellings. All tested Phytophthora isolates grew at an unusually high temperature of 39 °C, displayed similar or identical electrophoretic patterns of soluble proteins, and produced α1 hormone. The ability of one isolate of P. insolita to produce oospores was decreased and that of another was lost completely during storage. The results suggest that all isolates of the asexual Phytophthora sp. tested were Phytophthora insolita and were unable to produce oospores owing to a defect in the physiological process of sexual reproduction.Key words: asexual Phytophthora, Phytophthora insolita, high-temperature Phytophthora, sexual reproduction.

Some macadamia trees in commercial orchards in Hawaii showing quick decline syndrome had bleeding from the trunk. Phytophthora capsici was isolated from c. 67% of such sites, and was shown to kill branches of artificially-inoculated healthy macadamia trees. The pathogen was detected in both diseased and apparently healthy bark and isolated from wood 80 mm away from the bark. Results suggest that trunk infection by P. capsici may lead to girdling and rapid decline, and attract insects which then cause some bleeding by making wounds at the sites of recent infection.

Analysis of the isozyme patterns of glucosephosphate isomerase and peptidase from selfed progeny of isolates of Phytophthora infestans showed that some gave only a single slow or fast band, whereas others gave three bands, i.e., a slow, a fast, and an intermediate band. These results are consistent with the hypothesis that each locus is controlled by a single codominant gene. The single-banded pattern, either slow or fast, is due to homozygosity of one or the other allele, while the three-banded pattern is due to heterozygosity. The results also show that the genes for glucosephosphate isomerase and peptidase are not linked. Progeny from direct pairings between isolates with different homozygous genotypes for isozyme patterns consisted of selfs of both parents and hybrids, self of one parent and hybrids, and hybrids only. Key words: Phytophthora infestans, genetics, isozyme pattern.

Among 53 isolates of Phytophthora parasitica, Phytophthora palmivora, Phytophthora capsici, and Phytophthora cinnamomi tested, there was no difference in ability to produce lipase, phosphatase, or urea on solid media. However, only some isolates of P. parasitica and P. palmivora produced DNase or pectate transeliminase. The ability to produce pectin depolymerase or amylase varied greatly among isolates of some species. Zoospore progeny of A1 and A2 isolates of P. parasitica tested were relatively uniform and were similar to their parents. However, progeny from oospores produced by the same isolates as a result of hormonal stimulation displayed great variation in their ability to produce amylase. These results support the hypothesis that species of Phytophthora are diploid and indicate that amylase production in P. parasitica is controlled by heterozygous multiple genes.

The position of the antheridium on the oogonium of Phytophthora boehmeriae was greatly affected by culture media. Agar medium containing lecithin and basal salts and corn grain agar favored the production of paragynous antheridia, while corn meal agar, lima bean agar, vegetable juice agar, and tomato agar were conducive to formation of amphigynous antheridia. The effect of culture media on antheridial configuration was found to be due to nutrient concentration. A high concentration of nutrients in the media was favored the formation of amphigynous antheridia, and a low concentration of nutrients favored paragynous antheridia.Key words: antheridial configuration, oogonium, nutrients, Phytophthora boehmeriae.

A1 and A2 isolates of Phytophthora parasitica were exposed to light at different stages of sexual development to study the mode of action of light on sexual reproduction. Exposure to light during the process of sexual reproduction reduced the number of oospores produced to about 7% of that produced in darkness. Light was inhibitory to production of α hormones but not receptors of these hormones by both A1 and A2 isolates of P. parasitica. However, after being produced, α hormones were stable under light. The number of oospores produced was greatly reduced when A1 and A2 cultures were exposed to light during hormone induction of sexual reproduction but was not affected when the cultures were exposed to light during oospore formation after hormone induction. The results suggest that the effect of light on sexual reproduction in P. parasitica was mainly through inhibition of hormone production and hormone induction of sexual reproduction. Key words: Phytophthora parasitica, light effect, hormonal regulation.

Nuclei isolated from metalaxyl-resistant (Mr) protoplasts of Phytophthora parasitica were transferred into chloroneb-resistant (Cnr) protoplasts of the same species, with an average successful rate of 1x10(-4), using a selective medium containing both metalaxyl and chloroneb. No colonies appeared when self-fusion products of donor nuclei or recipient protoplasts were exposed to the selective medium. Similar results were obtained when the reciprocal nuclear transformation was performed. All the zoospore cultures produced by nuclear transformants contained both Mr and Cnr genes, demonstrating the occurrence of karyogamy since most zoospores are uninucleate. An average of 38% of zoospores produced by nuclear transformants were about twice the size of normal zoospores. Each large zoospore contained a single nucleus, also about twice the size of that in normal zoospores, indicating the occurrence of tetraploidy in these zoospores. The other 42% of zoospores were of normal size but contained both Mr and Cnr genes, suggesting the occurrence of mitotic crossing-over following the production of tetraploidy and the subsequent occurrence of diploidization during zoosporogenesis. For the first time, this study demonstrated the completion of events leading to a parasexual cycle following nuclear transfer.

Tomato agar and soybean agar were found to be comparable to or in some cases better than the popular V8 vegetable juice agar in supporting linear growth of Phytophthora cactorum, Phytophthora capsici, Phytophthora parasitica, Pythium aphanidermatum, and Pythium splendens ; sporangium production of P. capsici, P. palmivora, and Pythium splendens ; and oospore formation of P. cactorum, P. parasitica, Pythium aphanidermatum and Pythium splendens. These two media with readily accessible ingredients can be used to substitute for V8 juice agar in pathological and physiological studies of these two important groups of fungi in countries where V8 vegetable juice is not available or difficult to obtain.

SUMMARY: The germination pattern of six species of fungi on synthetic soil containing antibiotic-producing or non-antibiotic-producing micro-organisms was similar to that on natural soil. Sterilization by autoclaving destroyed the fungistatic effect of both natural and synthetic soils. Mixed microorganisms were more effective than bacteria, actinomycetes or fungi alone in inducing fungistasis in synthetic soil. The percentage germination of Exserohilum rostratum and Bipolaris maydis on both natural and synthetic soils increased with increase in the proportion of silica sand added. Bacteriostasis, actinostasis and fungistasis occurred concurrently in the synthetic soil, which also induced lysis of mycelia of Neurospora tetrasperma. Preincubation on natural or synthetic soil rendered nutrient agarose blocks incapable of supporting germination of nutrient-dependent fungi without reducing their ability to support germination of nutrient-independent fungi. Individual groups of micro-organisms were not as effective as mixed micro-organisms in causing diffusion of nutrients from agarose blocks to synthetic soil.

Summary: Streptomycin-resistant bacteria were added to natural soil and their numbers were determined at 0 and 24 h using soil extract agar supplemented with streptomycin. None of 10 rhizosphere and 10 non-rhizosphere bacteria tested increased in number in soil. Moreover, no increase was detected when six identified bacteria were added to soil. Agrobacterium radiobacter did not increase in any of seven soils tested. However, its population increased greatly in autoclaved soil as well as in nutrient-amended soil. The bacterium did not grow in distilled water, but grew abundantly in a nutrient solution. The characteristics of soil bacteriostasis appear to be similar to those of soil fungistasis.

Soils collected from five districts of Hawaii county were infested with Rhtzoctonia solani in small inoculum particles and successfully planted with radish to induce suppression, Suppressiveness was induced in some, but not all, replicates of all. soils. When fresh inoculum was added, suppressiveness was demonstrated in some, but not all, replicates of two soils, but not in the other three soils. Acidity of soil was not important in successful induction of suppression. Characteristics of induced suppression in soil from one site (S. Kohala) were further investigated. Reduction of microbial population by heat treatment of suppressive soil completely nullified its inhibitory effect. The populations of actinomycetes, fungi in general and Trichoderma spp. in suppressive and conducive soil were not significantly different. However, the population of bacteria in suppressive soil was almost four times higher than that in conducive soil. The survival time of R. solani in suppressive soil was shorter than that in conducive soil. Hyphae of R. solani also lysed faster in suppressive soil than in conducive soil. It is suggested that suppressiveness of the South Kohala soil created by monoculture is due to enhanced competitive pressure generated by an increased bacterial population, which in turn causes the rapid autolysis of R. solani hyphae.

Germination of nutrient-amended chlamydospores of the three formae speciales of Fusarium oxysporum tested were inhibited in a suppressive soil collected from central Taiwan. The suppressive soil released a volatile substance when moistened with alkaline solution. The inhibition spectrum of the volatile substance was different from that of the suppressive soil. The inhibitory effect of the suppressive soil was greatly reduced when it was heat-treated for 30 min at 40°C or higher. The inhibitory effect of the heat-treated suppressive soil was restored after infestation with 1% conducive or suppressive soil for 14 days. However, infestation of heat-treated conducive soil even with 1% suppressive soil did not render it suppressive. Amendment of suppressive soil with rose bengal, streptomycin or Rubigan completely or partially reduced the inhibitory effect. Increasing the total population of indigenous microorganisms in conducive soil by amendment with rice germ or soybean meal to about the same level as that in suppressive soil did not render it suppressive. Results suggest that a combination of biotic and abiotic factors is responsible for the inhibitory effect of the suppressive soil. Zusammenfassung Charakteristische Merkmale der Hemmung eines Taiwanbodens, der suppressiv gegen Fusarium oxysporum f. sp. raphani ist Die Auskeimung nahrstoffverbesserter Chlamydosporen der drei getesteten formae speciales von Fusarium oxysporum wurde in einem aus Zentraltaiwan gesammelten suppressiven Boden gehemmt. Nach Anfeuchtung mit einer alkalischen Losung wurde erne fluchtige Substanz freigesetzt. Das HemmuDgsspektrum der fluchtigen Subsranz war mit dem des suppressiven Bodens nicht identisch. Die hemmende Wirkung des suppressiven Bodens wurde nach einer 30minutigen Hitzebehandlyng bei 40°C oder hoher stark reduziert. Die hemmende Eigenschafi des hitzebehandclten suppressiven Bodens wurde 14 Tage nach l%iger Beimischuing eines fordernden oder suppressiven Boden5 wieder hergestellt. Dagegen konnte eine 1 %ige Beimischung eines suppressiven Bodens zu einem hitzebehandelten fordernden Boden diesen nicht suppressiv machen. Eine Verbesserung des suppressiven Bodens mit Rosenbengal, Streptomycin oder Rubigan verursachte eine ganze oder partielle Aufhebung der hemmenden Wirkung. Auch die Erhohung der Mikroorganismenpopulation eines fordernden Bodens, um diese der eines suppressiven Bodens ungefahr anzugleichen, durch eine Verbesserung mit Reiskeim- oder Sojabohnenmehl machte ihn nicht suppressiv. Die Ergebnisse deuten darauf hin, das eine Kombination biotischer und abiocischer Faktoren fur die hemmende Wirkung eines suppressiven Bodens verantwortlich ist.

When Phytophthora colocasiae (A2) was used as a hormone producer and P. parasitica (A1) as a hormone receptor, no sex organs of P. parasitica were observed in matings lasting for 7 h, but the amount of hormone produced was sufficient to stimulate the production of 341 oospores cm-2 6 d later. The maximum induction of sex organs was reached in matings lasting 48 h. Hormone production was inhibited by light, but the effect of light on oospore development was small. Temperatures of 10 and 15 °C inhibited the growth and hormone production by P. colocasiae, and prevented P. parasitica from forming new sex organs after stimulation by hormone. The effect of temperature on hormone production differed from that on oospore formation. Hormone production was poor at 30 °C, but oospore development was good at this temperature.

Several species of the genus Phytophthora of the order Peronosporales in Oomycetes cause plant diseases of worldwide importance. Among them are late blight of potato, caused by P. infestans; black pod of cacao, caused by P. palmivora; and root rot of avocado, caused by P. cinnamomi (68). Some species of Phytophthora such as P. cactorum, P. megasperma, and P. citricola are homothallic and are capable of completing their life cycle by single isolates. However, other species, including P. infestans, P. palmivora, and P. cinnamomi, are heterothallic and require the presence of opposite mating types known as A 1 and A 2 for the formation of the sexual stage of the life cycle (21, 54). Heterothallic species of Phytophthora are unique in that sexual reproduc­ tion readily occurs even between morphologically and physiologically distinct species, for example, between the primitive soil-borne P. cinnamomi and the advanced air-borne P. infestans (35, 54). This fact suggests that chemical stimulation may be involved in oospore formation during mating, a hypoth­ esis originally proposed by Ashby in 1929 (6). Although Ashby did not exclude the occurrence of cross fertilization, he subordinated it to selfing.

Phytophthora cinnamomi-suppressivc soils were found to be widely distributed in nature. About 40 % of soil samples collected from locations with different vegetation, soil type or elevation throughout the island of Hawaii were suppressive to chlamydospore germination of Phytophthora cinnamomi. Soil samples collected from the same general areas varied greatly in degree of suppressive-ness to P. cinnamomi, ranging from conducive to strongly suppressive. Among the 155 soil samples tested, those with pH close to 4 or 8 tend to be more suppressive to P. cinnamomi than those with pH close to 6. Zusammenfassung Die Verteilung von Phytophthora cinnamomi-suppressiven Boden in der Natur Festgestellt wurde eine in der Natur breit verbreitete Verteilung von Phytophthora cinnamomi-suppressiven Boden. Zirka 40% aller Bodenproben, die an verschiedenen Standorten mit unterschiedlichem Pflanzenwuchs, Bodentyp oder bei unterschiedlichen Hohenlagen auf der ganzen Hawaii-Insel gesammelt worden waren, zeigten eine Suppressivitat gegenuber der P. cinnamomi- Chlamydosporenkeimung. In einer Region gesammelte Bodenproben wiesen ganz unterschiedliche Suppressivitaten gegen P. cinnamomi auf, die sich zwischen forderlich und stark suppressiv bewegten. Unter den 155 untersuchten Bodenproben wurde eme ausgepragte Suppressivitat bei den Proben, die pH-Werte um 4 oder 8 hatten als bei denen mit einem pH-Wert um 6, ermittelt.

Summary: Zoospores of Phytophthora palmivora suspended in water aggregated within 2 min at 16°C. Individual spores in aggregates were separate and actively swimming. The optimum temperature for aggregation was 16°C. Aggregate formation depended on zoospore concentration and depth of suspension.