Potato blight Phytophthorainfestans

This fungus is a member of the Zygomycota division of fungi (see p74).

Damage. This important disease is a constant threat to potato production; it caused the Irish potato famine in the nineteenth century. The first symptoms seen in the field are yellowing of the foliage, which quickly goes black and then produces a white bloom on the under surface of the leaf in damp weather. The stems may then go black, killing off the whole plant. The tubers may show dark surface spots that, internally, appear as a deep dry red-brown rot. This fungus may attack tomatoes, the most notable symptom being the dark-brown blisters on the fruit (Figure 15.5).

Life cycle. The fungus survives the winter as mycelium and sexual spores in the tubers (see Figure 15.4, p235). The spring emergence of infected shoots results in the production of asexual spores.

  1. The spores are spread by wind and land on potato leaves or stems. They can, after infection, result in a further crop of spores within a few days under warm, wet weather conditions. The disease can spread very quickly. Later in the crop, badly infected plants may have tuber infection as rainfall washes spores down into the soil.
  2. The amateur gardener should use clean seed, choose resistant varieties, and apply a protective fungicide such as copper sulphate (Bordeaux mixture) before damp conditions appear. The professional grower similarly needs to use clean seed. Removal and herbicidal destruction, (e.g. using dichlobenil), of diseased tubers from stores or 'clamps' prevents disease spread. Knowledge of the disease's moisture requirement leads to better control. DEFRA are able to measure both the duration of atmospheric relative humidity greater than 92 per cent, and mean daily temperatures greater than 10°C (together known as critical periods), and to issue forecasts of potato blight outbreaks. In this way, protectant sprays of chemicals such as mancozeb can be applied before infection can take place.

Resistant potato cultivars prevent rapid build-up of disease, although resistance may be overcome by newly occurring fungal strains. Early potato cultivars usually complete tuber production before serious blight attacks, while main-crop top growth may deliberately be killed off with foliage-acting herbicides, such as diquat, to prevent disease spread to tubers.

A curative, systemic fungicide such as benalaxyl penetrates the leaf and kills the infecting mycelium. This ingredient has within its formulation a protectant ingredient (in this case mancozeb) to kill off most germinating spores and thus reduce the development of fungus resistance to the systemic fungicide.

Potato Damaged Fungicide
Figure 15.6 Leaf symptoms of cabbage downy mildew on stock. Lower leaf symptoms (top), upper leaf symptoms (bottom)

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