Phomopsis sclerotioides

Black root rot of cucurbits

General

Phomopsis sclerotioides is a plant pathogen infecting cucurbits, causing black root rot of cucurbits.

Life cycle and appearance of Black root rot of cucurbits

Phomopsis sclerotioides survives in the soil and is a strong saprophytic competitor, which means it can rapidly colonize ‘clean’ or sterilized soil and substrate. It aggregates, so disinfection needs to be applied thoroughly and evenly. Phomopsis sclerotioides forms sclerotia which can survive adverse conditions for years in the soil, but little else is known about the life cycle. The fungus is dispersed by water, air and soil particles.

Damage symptoms

Phomopsis sclerotioides causes grey-black spots with a dark demarcation line on cucumber roots. The lesions are slightly sunken and are mostly formed around the point of attachment of lateral roots and root hairs. The lesions may grow and encircle the root. The roots become yellow-brown to black and ultimately die. Black lesions with fruiting bodies occur more commonly at temperatures of approximately 20 °C whereas at 10 °C the lesions are light-brown. The lesions can be enclosed by pseudostromata: dark coloured tissue that looks like black stripes.

The disease is usually restricted to a few of the roots but in severe cases all roots may be infected. At the final stage of the disease, only vascular tissue remains because the cortex (outer layer) has rotten away. Infection in the early stages of the crop results in stunted growth and smaller leaves with an intense green colour. Severe infection in this stage may kill the plants. In case of a less severe infection, the vines may wilt at a late stage of the crop, but when water supply is in any way limited during earlier periods of high evaporation, wilting may occur sooner. In wilted plants, the crown first gets a water-soaked appearance, turning light-brown and sunken, later black with amber-coloured exudations of spores. In the early stages of wilting it is hard to distinguish from other wilt diseases.

The fungus can also cause black fruit rot.

How to prevent Black root rot of cucurbits

  • Test for the presence of Phomopsis sclerotioides with molecular tools before planting
  • Use resistant rootstock of Cucurbita ficifolia
  • Apply steam sterilization to the soil/substrate
  • Fumigation with soil disinfectants is only effective when applied evenly
    Note: since this fungus can rapidly re-colonize the soil/substrate, disinfection should be applied at least annually
  • Disinfect nutrient solution and irrigation water

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