Rhizoctonia solani as a component in the bottom rot complex of glasshouse lettuce
The basal parts of maturing glasshouse lettuce can be attacked by several soil fungi, which cause bottom rot. Until recently quintozene was generally applied against this disease complex. The study of the causal fungi - especially Rhizoctonia solani - and their control was undertaken in view of the need for quintozene replacing fungicides.A survey revealed that Botrytis cinerea was the most frequently observed pathogen, especially in winter crops. The incidence of Sclerotinia minor, Sclerotinia sclerotiorum, Pythium spp. and Rhizoctonia solani was much lower. These fungi were more prevalent in autumn and late spring crops than in winter crops. Many of the attacked heads were infected by more than one pathogen. On an average disease severity caused by R. solani was the highest and by Pythium spp. the lowest. Trials showed that on untreated areas the average loss of lettuce heads of export quality class caused by bottom rot was 17 per cent, whereas on plots treated with appropriate fungicides this loss was 5 per cent.For the detection of R. solani in the soil a relatively simple and fast paper disk bait method was improved and standardized. In addition a time saving quantitative technique was developed by plating out sieved soil clumps on a selective medium.The paper disk method proved that the horizontal dispersion of R. solani propagules in the soil of lettuce glasshouses is not random, but clustered. This pattern could be characterized by the negative binomial distribution. Propagules were found throughout the whole ploughlayer but the inoculum density decreased remarkly as the depth of sampling increased.The average inoculum density on 62 holdings was low: 0.6 propagule of R. solani per 100 g of soil. The individual glasshouse values were related to the period of sampling, soil disinfestation and the use of quintozene.During winter crops the inoculum density remained low, in spring time the values were higher, especially at the end of the cropping period. An attack of R. solani in most lettuce crops can be characterized as a "simple interest disease" sensu Vanderplank. This implies that the reduction of the initial inoculum in the soil is the most important factor in the control strategy.On a log-log scale there appeared to exist a linear relationship between the initial inoculum density of R. solani determined with the paper disk method and the infections of lettuce heads. This indicates that this method of detection has a disease forecasting value. A tentative threshold value of initial inoculum density of 0.2-0.5 propagules per 100 g of soil was established, if the value is less than this no economic loss need be expected.Most of the R. solani isolates from lettuce leaves that were examined showed that the minimum temperature required for infection was at least 9°C.The incubation period at this temperature was from 11 - 15 days, whereas at 20°C less then 3 days. The linear extension of the lesions on infected leaves was at 20°C about 8 mm per 24 hour. These data confirm the observations that on lettuce, ,cropped during winter months with a low temperature regime, the attack is minimal, and that in late spring crops with periods of high temperature the attack can progress very quickly. Isolates were obtained with specific low temperature requirements from a glasshouse with a winter crop which had been severely attacked.Rhizoctonia solani in lettuce appeared to be hygrophylic, but free water is not necessary for the initiating of the infection process.All seventeen glasshouse crops tested, in a host range experiment, were attacked by R. solani.In isolates obtained from diseased heads, three out of four known European anastomis groups of R. solani occurred. The other, anastomis group 3 ("potato group") was only found once among isolates from soil.The dicarboximides, iprodione and vinclozolin, have proved in field and in vitro experiments, to be good replacements for quintozene to control attacks of B. cinerea, R. solani and Sclerotinia spp. The infection by Pythium spp. appears to increase when quintozene and dicarboximides are used. Further research is necessary in order to establish whether the chemical control of Pythium spp. will be required economically.Soil disinfestation with methyl bromide caused a large reduction of the inoculum density of R. solaniFinally it has been discussed that the control of bottom rot should be optimized by integration of the culture of the more tolerant "upright" cultivars, the management of appropriate cultural measures and a reduced dose of fungicides.
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
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Landbouwhogeschool
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| Subjects: | deuteromycotina, greenhouse horticulture, lactuca sativa, lettuces, moniliaceae, plant pathogenic fungi, soil fungi, bodemschimmels, glastuinbouw, plantenziekteverwekkende schimmels, slasoorten, |
| Online Access: | https://research.wur.nl/en/publications/rhizoctonia-solani-as-a-component-in-the-bottom-rot-complex-of-gl |
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