Sclerotia disease in cabbages can, in severe cases, wipe out an entire cabbage crop. It’s a sporadic problem and by understanding the life cycle of the disease, you can minimise its effects and gradually get rid of it.
Sclerotia also attacks other plants such as green beans, and is a major problem in lettuce in some parts of the world. Sclerotia manifests as a cottony growth of white fungus or mycelium which just about covers the entire plant. In an advanced stage, hard, black bodies that resemble flattened mouse droppings form in this mass of white fungus. These bodies are called sclerotia. These sclerotia represent the dormant phase of the fungus and can survive in the soil for a number of years. As cultivation brings them closer to the soil surface, they enter into the next phase when favourable weather conditions prevail.
During cool, moist conditions, match head-sized “mushrooms” grow from these. They are cream to tan coloured and are called apothecia. When dry conditions set in and humidity drops, they pump out huge numbers of spores that drift in the wind and lodge in the crop. These spores can survive for a few weeks and germinate in the presence of free water. At this stage they need dead or decaying material to start growing. Injury caused by moth larvae is a good starting point. But they also develop where the leaves meet the head and have been known to develop without apparent dead material. The worst case of sclerotia I have seen was in a crop of cabbage planted on land where sunflowers were once cultivated.
If the disease is expected, spray with Rovral or Sumisclex and ensure that pest control is good. When the odd plant is found with the disease, remove it and destroy it to break the life cycle.
Farmers tend to find the odd infected plant, thinking that the damage is so slight that it is not worth worrying about. Millions of spores can be produced from sclerotia of one plant. Therefore, the extra effort is well worth the trouble.
White blister is generally not a serious disease in cabbages, but is often a serious disease of radishes and many weeds including Shephards Purse. It develops in cool, wet weather and spores are wind-blown. The disease is easily identified by lighter-coloured, raised lesions that are noticeable on top of the leaf. When the leaf is turned over, white pustules can be seen. If the disease is severe enough to justify control, an application of Ridomil will stop its progressions. – Bill Kerr Contact Bill Kerr on (016) 366 0616 or e-mail [email protected]. |fw