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Because biological control ofRhizoctonia solani in potato with conidial suspensions of the mycoparasiteVerticillium biguttatum was often less successful in sandy soils than in loamy soils, we examined soils of potato fields for the presence of organisms destructive to conidia ofV. biguttatum.

The effects of biological, chemical and integrated control on the formation of selerotia ofRhizoctonia solani on new potato tubers were studied in experimental fields. Sprouts of seed tubers, sprouted in daylight, were inoculated withVerticillium biguttatum, an ecologically obligate mycoparasite ofR. solani. Fungicides were mixed with the top soil. Biological control byV. biguttatum increased the percentage of harvests with less than 5% loss due to grading from 24% (non-inoculated) to 56%. Fungicides at the recommended rates usually gave good results but lower doses were less effective in sand than in loam soils. When fungicides were combined with biological control, the results were often, better: harvests with less than 5% sorting loss increased from 56 to 81%. Integrated control with pencycuron at 25% of the recommended rate was about equal to chemical control with pencycuron at full rate. The advantages of integrated control are discussed.

The position of plants withRhizoctonia solani sclerotia (black scurf) on progeny tubers was mapped for an experimental field at Haren where potatoes were grown continuously and in rotation with other crops for five successive years, and for another field at Borgercompagnie with a 1∶2 frequency of potatoes during three potato crops. Initially, the distribution of plants with black scurf on both fields was rather dense and homogeneous. In the following years the distribution became heterogeneous and patchy. The local decline ofR. solani AG 3 (the common potato pathogen) in Haren was apparently caused by an unknown factor selectively suppressingR. solani AG 3, while simultaneouslyR. solani AG 5 increased in mass. This AG 5 type proved to be an inferior competitor of AG 3 on the potato plant in a laboratory experiment. The specificR. solani antagonistVerticillium biguttatum did not play a role. A similar factor could have reduced the formation of black scurf in the experimental field at Borgercompagnie, whereV. biguttatum was also too infrequent to account for the decline.R. solani AG 5 was not present here and could not indicate the presence of a selective factor against AG 3.

Jager, G., Velvis, H., Lamers, J.G., Mulder, A. and Roosjen, Js., 1990. Biological, chemical and integrated control of Rhizoctonia solani in potato. The effects of biological, chemical and integrated control on the formation of sclerotia of Rhizoctonia solani on new potato tubers were studied in experimental fields on neutral marine loams and slightly acid sands. Sprouts of seeds sprouted in daylight were inoculated with conidia of Verticillium biguttatum , an obligate mycoparasite of R. solani. Fungicides at the desired dosage rates were sprayed on the soil and mixed with it. Biological control increased the percentage of harvests with less than 5% loss due to grading from 24% (non-inoculated) to 56%. Chemical control at the recommended dosage rate usually gave good results. Lower doses were less effective in sand than in loam soils, but combined with biological control the results were often good: harvests with less than 5% loss due to grading increased from 56 to 81%. Integrated control with 1/4 Monceren was about equal to chemical control with 1/1 Monceren. The results were obtained under conditions favouring formation of sclerotia on tubers. Better results are expected with improved methods of harvesting. Advantages of integrated control are discussed.

In field experiments in 1983, inoculation of seed tubers with three isolates ofVerticillium biguttatum, separately or mixed, was successful in reducing the amount of sclerotia ofRhizoctonia solani formed on new tubers. In holocene marine soils the sclerotium index (s.i.) was on average 50% of that of the harvest from untreated tubers.

Antagonism of isolates ofAzotobacter chroococcum toRhizoctonia solani on agar plates was studied, and isolates were tested for their ability to controlR. solani infection of potato sprouts in sterilized and unsterilized soil. The degree of antagonism exhibited varied strongly among the isolates and was also found to be temperature-dependent.

Inoculation of seed potatoes withVerticillium biguttatum and three other hyperparasitic fungi, alone or in combination, resulted in statistically significant reduction of infestation of potato plants byRhizoctonia solani. Gliocladium roseum, Trichoderma hamatum andHormiactis fimicola did not show prolonged protection againstR. solani under farming conditions.H. fimicola, however, inhibited mycelial growth ofR. solani in vitro, particularly in the lower temperature range whereV. biguttatum did not show any growth. Combining these two antagonistic fungi may be advantageous as they cover the entire temperature range in whichR. solani is active. At the end of the vegetation period,V. biguttatum was superseded byG. roseum. Production of sclerotia on newly formed tubers from seed potatoes inoculated withV. biguttatum (alone or in combination with the other three antagonists) was significantly reduced. Inoculative van pootaardappelen metVerticillium biguttatum, apart of sament met drie andere hyperparasieten, had een gunstig effect op het onderdrukken vanRhizoctonia solani op de plant;Trichoderma hamatum, Gliocaldium roseum enHormiactis fimicola, ieder apart toegediend, boden de plant op lange termijn geen bescherming tegenR. solani onder praktijkomstandigheden.H. fimicola bleek bij lage temperatuur, waarbijV. biguttatum geen groei-activiteit meer vertoonde, op hyfen en sclerotien vanR. solani te kunnen groeien. Toepassing van deze schimmel enV. biguttatum in een gemengde inoculatie zou over een breder temperatuurtraject effectief kunnen zijn. Tegen het einde van het groeiseizoen vond er op de ondergrondse plantedelen een verschuiving plaats, waarbijG. roseum meer op de voorgrond trad. Verondersteld wordt dat de afname vanV. biguttatum op stolonen hiervan een gevolg was. De produktie van sclerotia op nieuwe aardappelen afkomstig van metV. biguttatum behandeld pootgoed (alleen of met andere hyperparasitaire schimmels) bleek sterk verminderd te zijn. Vooral dit gegeven maakt de biologische bestrijding vanR. solani interessant:V. biguttatum blijkt ook op lange termijn effectief te zijn.

Gliocladium roseum was found to be the most common and probably the most effective mycoparasite in potato fields in the northern parts of the Netherlands. It is able to parasitize and kill living hyphae at temperatures of 12°C and higher. Sclerotia ofR. solani are often infected and killed by this fungus under suitable conditions, i.e. at temperatures of 16°C and more. Killing of sclerotia by other antagonistic organisms was also observed. It is also shown by not parasitic fungi and is caused by toxins produced by the antagonist.

The subterranean parts of 1585 wild plants (weeds) belonging to 52 species, found in potato fields in the northern part of the Netherlands were examined for the presence of Rhizoctonia solani. The fungus could be isolated from 30 plants belonging to 12 species. Of these isolates 62070 proved to be pathogenic to potato sprouts. Some species seemed more frequently colonized by R. solani than others, particularly Solanum nigrum, Elytrichia repens and Matricaria recutita. On some common weeds, viz. Chenopodium album and Poa annua, no R. solani was observed, while about 1~ of all Polygonumpersicaria were infected. Disease symptoms were only observed once on Elytrichia repens as 'sharp eyespots', presumably caused by R. cereal&. The incidence of R. Solani on weeds increased markedly towards the end of the growing season.

A common mycoparasite,Verticillium biguttatum, was found to kill sclerotia ofRhizoctonia solani placed on an inert material (perlite) as well as in soil at 15°C and 20°C, but not at 10°C. Compared with the effectivity ofV. biguttatum, that ofGliocladium roseum, Gliocladium nigrovirens, Hormiactis fimicola andTrichoderma hamatum on sclerotia was only low. In laboratory experiments, treatment of sclerotia-bearing seed potatoes withV. biguttatum reduced disease symptoms in the first stage of growth of the potato plant.V. biguttatum was found to occur on the subterranean part of the potato plant. On untreated plants the surface of the sprouts was colonised byV. biguttatum originating from the soil, presumably partly in response to the presence ofR. solani mycelium. In a preliminary field experiment,Verticillium treatment did not reduce symptoms on the stem. However, there was a marked reduction in sclerotium formation on the newly formed potato tubers. This offers perspectives for a commercial use ofV. biguttatum in the control ofR. solani. Sclerotien vanRhizoctonia solani, die bij het rooien van aardappelen op de knollen zitten, blijken veelvuldig geinfecteerd te zijn met de schimmelVerticillium biguttatum. In dit onderzoek is aangetoond datV. biguttatum in staat is de sclerotien vanR. solani te doden. Dit effect wordt bereikt zowel wanneer de beente sclerotien geincubeerd worden op perliet als in grond.V. biguttatum kan zich ook in het bodemmilieu goed op de sclerotien handhaven. In vergelijking met de doding van sclerotien doorV. biguttatum is het effect van andere bekende mycoparasieten alsGliocladium roseum enHormiactis fimicola vrij gering, en dat vanGliocladium nigrovirens enTrichoderma hamatum zelfs te verwaarlozen. De temperatuur blijkt voor de activiteit vanV. biguttatum een belangrijke factor. Pas tussen 10 en 15°C treedt groei en doding van sclerotien op. Onder laboratoriumomstandigheden bij 15°C resulteerde een behandeling van sclerotiendragende pootaardappelen in een vermindering van deRhizoctonia-aantasting van de jonge spruiten. De ontwikkeling vanRhizoctonia-mycelium op de ondergrondse stengels was bij de onbehandelde aardappelen aanzienlijk groter. V. biguttatum groeide bij de behandelde serie met de spruiten mee. Ook bij de onbehandelde objecten trad een sterke kolonisatie van het stengeloppervlak doorV. biguttatum vanuit de grond op. HoewelV. biguttatum ook zonderR. solani voorkomt als bewoner van aardappelstengels, lijkt het frequente voorkomen vanV. biguttatum in aanwezigheid vanR. solani te duiden op een interactie op het stengeloppervlak. In een kleine veldproef kon vermindering van de stengelaantasting door behandeling van het pootgoed metV. biguttatum niet worden aangetoond. De onderdrukking van de sclerotiumbezetting op de eindoogst bleek evenwel aanzienlijk. In latere veldproeven, waarover nog gepubliceerd zal worden, werd ook een effect op de stengelaantasting geconstateerd. De uitkomsten bieden perspectief voor de toepassing vanV. biguttatum als biologische bestrijder vanR. solani in de praktijk. De vraag ofV. biguttatum de aardappelplant ook bescherming biedt tegenRhizoctonia-infectie vanuit de grond dient nader te worden onderzocht.

A search was made forRhizoctonia solani-suppressive soils by establishing many small experimental plots, half of which were planted withRhizoctonia-infected seed potatoes and the other half with disinfected seed stock. The sclerotium index of the harvested tubers was compared witht that of the seed potatoes. In suppressive soils, the sclerotium index of the harvest is much lower than that of the seed potatoes.

Addition of live mycelium ofRhizoctonia solani to three slightly, acid sands collected in winter from a field after a previous potato crop resulted in accumulation of hyperparasites and less infestation of sprouts of infected seed potatoes.

Verticillium biguttatum isolate M73, when inoculated on the seed tuber, was found to protect young potato sprouts againstRhizoctonia infection from the soil and also, as was demonstrated before, against infection from sclerotia on seed tubers. The experiments were performed under laboratory conditions and lasted a few weeks. V. biguttatum colonizes the surface of the sprouts and may be regarded as a natural inhabitant of the surface of sprouts and presumably also of stems and stolons in a later stage. This fungus may be of value for biological control ofR. solani. Isolaat M73 van de schimmelVerticillium biguttatum, indien geent op de pootknol, blijkt de aantasting van aardappelspruiten doorRhizoctonia solani te kunnen verminderen. Zowel de aantasting vanuit de grond en, zoals reeds eerder aangetoond, vanaf sclerotien op de pootknol worden minder. Verticillium biguttatum koloniseert het oppervlak van de spruiten en kan beschouwd worden als een natuurlijke bewoner van het oppervlak van spruiten en wellicht ook van stengels en stolonen in een later stadium. In kort durende proeven, als hier beschreven, lijktV. biguttatum goed bruikbaar voor een biologische bestrijding vanR. solani.

The seed potatoes used in these experiments had been grown in a slightly acid pleistocene sandy soil or in a marine, holocene sandy loam. They were free of sclerotia ofR. solani or lightly or moderately speckled with them. Seed potatoes from the sandy soil produced plants that suffered less fromRhizoctonia than plants from seed potatoes that had been grown on the marine sandy loam. Similarly harvested tubers had, in a non-conducive soil and in conducive soils with a (very) low inoculum density ofR. solani, fewer sclerotia when they came from seed potatoes grown in an acid sand.

Experiments in the laboratory and on farms with potato tubers in storages are described in which sclerotia of Rhizoctonia solani were inactivated after inoculation of infected tubers with a suspension of conidia and hyphal fragments of Verticillium biguttatum

The presence of the pathogenic fungus Rhizoctonia solani in potato fields and the expression of damage to plants is often irregular and patchy.

The viability of sclerotial and hyphal inoculum of R. solani artificially added to different Dutch soil types, can be reduced rapidly and substantially as a consequence of attack by the mycoparasite Verticillium biguttatum, V. biguttatum was found to be the main organism responsible for this reduction observed in laboratory experiments. No other microbial antagonists accumulated on the propagules. In one case viability reduction was apparently due to fungivorous nematodes. Inoculum, however, that had escaped from V. biguttatum , showed no noticeable decline during 6 months at 20°C. Sclerotial inoculum, only incidentally infected by V. biguttatum , showed a gradual decline during 2 yr in five soil types under field conditions. The reduction could not be ascribed to other biological factors examined.