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In this article we will discuss about the powdery mildew of peas caused by fungi.
Introduction to the Powdery Mildew of Peas:
The powdery mildew of peas is worldwide in occurrence causing serious losses to the crop. The disease is considered to be much more serious than the downy mildew of peas.
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Physiologic races of the fungus attack beans, urd, lucerne, coriander, turnip, cabbage and many other plants. In India, the disease generally appears during November-December.
Effect of Powdery Mildew Disease:
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The disease appears in epiphytotic form in the plains almost every year. The disease is worst in dry weather causing great loss to the crop, although varieties maturing in January escape the disease.
The loss to the crop is proportional to the intensity of the disease. In a 100 percent infected crop, the loss is estimated to range between 20-25 percent. Munjal et al. (1963) have estimated the loss in pod number to 21-30 percent and reduction in pod-weight by about 26-50 percent.
Symptoms of Powdery Mildew Disease:
First symptoms appear on the leaves in the form of white floury patches on both sides of leaves. The disease then spreads to other green parts of the plant such as tendrils, pods, stems etc. The patches on the leaves originate in the form of minute discoloured specks from which powdery mass radiates on all sides.
In the advanced stages of the disease large areas of the host get coveted with white floury patches. Infected plants impart dirty appearance. In extreme severe infections the infected leaves are shed leaving stem devoid of the leaves.
Causal Organism:
The causal organism is Erysiphe polygoni DC, although E. pisi and E. cichoracearum have also been reported on peas, but E. polygoni is more destructive. The fungus is an obligate parasite having septate, hyaline, profusely branched superficial mycelium sending finger shaped haustoria to the host cells.
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Disease Cycle:
The primary infection takes place through ascospores released in soil as a result of disintegration of the wall of cleistothecium. Under favourable conditions, the ascospores come in contact with the lower most leaves of the host, germinate by germ tube and cause primary ‘infection’.
From the lower most leaf, the infection spreads to other leaves. Later the symptoms in the form of white powdery mass appear. These patches contain conidia and conidiophores. The primary infection may also come from the conidia produced on other hosts as the causal organism E. polygoni has a large host range.
The secondary infection of the host takes place by conidia which are produced in large numbers and are disseminated by wind. Upon reaching the host the conidia germinate and cause infection.
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During the growing season, more than one secondary infection may occur because the establishment of infection and conidia production is a quick process. This results in severe large scale infection on the crops.
The cleistothecia-the perennating structures are produced on fallen infected debris in soil. These remain buried in the soil waiting for the favourable conditions. These contain asci and ascospores which serves as primary inoculum of the disease infection.
Control Measures of Powdery Mildew Disease:
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(i) Field sanitation, rotation of crop and destruction of diseased plant debris may effectively control the disease.
(ii) Sulphur dusting @ 25-30 Kg/hectare gives effective results. Only one dusting is sufficient.
(iii) Spraying with Karathane (0.2%), Elosal (0.5%) and Morocide (0.1%) have been found effective in controlling the disease.
(iv) Early maturing and resistant varieties like P185, 388, 61113, 1683, 6588, Freazer-656, T-10, T-163 and IC775 should be cultivated.
