By H. Fukada and L. J. du Toit
Cause Diaporthe sclerotioides (syn. Phomopsis sclerotioides) is a soilborne fungus that causes root rot of cucumber and other cucurbits. The fungus forms pseudosclerotia and pseudostromata in infected roots that can survive several years in soil, even in the absence of cucurbit plants. The disease is a problem in cucumber production in Europe, Asia, and Canada. Cucumber root rot was first found in the US in cucumber fields in northwestern Washington in 2012 and 2017. Cucumber black root rot is favored by acid soils (pH < 6.5), water deficit, heat stress, and fruiting. Most cucumber varieties are highly susceptible. Pumpkin and squash are much more tolerant than cucumber.
Symptoms Cucumber plants start to wilt late in the season, usually after fruit have formed. Wilting develops very rapidly over the whole plant, usually without initial yellowing (chlorosis) or necrosis of the leaves. Symptoms on roots include salmon-colored to gray or brown lesions, sometimes with dark-black lines (pseudostromata) that demarcate infected areas of roots. The crown and tap root of infected plants become dry and corky and, when severe enough, plants start to wilt very rapidly. Root symptoms, including the dark lines and pseudosclerotia, may only be observed after the onset of severe wilting. Distinct, rectangular, black pseudosclerotia can be observed in individual cells of the cortex, creating a checkerboard appearance. The fungus occasionally forms black pycnidia in infected roots and crowns.
Cultural control
- Do not plant cucurbits for consecutive years in the same field.
- Use limestone to increase the soil pH > 6.5.
- Avoid moisture stress.
- Avoid moving soil from infested fields on equipment, shoes, vehicle tires, etc.
- Remove diseased plants after harvest.
- Rootstock of Cucurbita ficifolia is less susceptible than Cucumis sativa.
Chemical control
Soil fumigation can reduce the population of propagules (pseudosclerotia and pseudostromata) of D. sclerotioides surviving in soil. Similarly, steam sterilization of soil can control black root rot, but the fungus can reinvade treated soil rapidly.
References Bruton, B.D., and Biles, C.L. 2017. Phomopsis black root rot of cucumber. p 42-43 in Compendium of Cucurbit Diseases and Pests, Second Edition. A.P. Keinath, W.M. Wintermantel, and T.A. Zitter, (eds.). St. Paul, MN: American Phytopathological Society.
Fukada, H., Derie, M.L., Shishido, K., and du Toit, L.J. 2018. First report of Diaporthe sclerotioides, causal agent of black root rot of cucumber, in Washington state. Plant Disease 102.
Shishido, K., Murakami, H., Kanda, D., Fuji, S., Toda, T., and Furuya, H. 2016. Effect of soil inoculum density and temperature on the incidence of cucumber black root rot. Plant Disease 100:125-130.