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In this article we will discuss about:- 1. Symptoms of Downy Mildew of Pea 2. Causal Organism of Downy Mildew of Pea 3. Disease Cycle 4. Control.
Symptoms of Downy Mildew of Pea:
Yellow to brown scattered patches of discoloured areas appear on the upper surface of the leaflets and stipules. Simultaneously with this, downy growth in patches of variable size are visible on the under surface corresponding with the lesions of the upper surface. The downy coating is whitish when young, but the colour gradually turns greyish-violet with age.
This growth is composed of branches sporangiophores, arising from the mycelium of the fungus which has entered within the leaf tissues. Sporangia are borne at the tips of the ultimate branches. The infected leaflets and stipules become reduced in size with their margins curled downwards. The infected areas gradually develop into elongated blotches, and often to irregular spots.
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With the spread of infection, blotches also appear on the pods. When young, the blotches on the pods are pale-green, more or less elliptical to irregular, but gradually the blotches turn dark to bright-brown, mottled with light-green islands. Blotch followed by the green island effect is a typical symptom of the disease.
Seeds lying corresponding to the infected tissues of the pods abort and become very much reduced in size. Conidia are very seldom produced on the pods.
The Causal Organism of Downy Mildew of Pea:
Downy mildew of pea is caused by Peronospora pisi Sydow, whose mycelium consists of aseptate branched hyaline hyphae, confined to the intercellular spaces of the host tissues. The hyphae produce branched, finger- shaped haustoria which penetrate into the adjoining cells of the intercellular spaces (Fig. 359B).
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The sporangiophores arise directly from the internal hyphae and emerge in clusters from the stomata of the under surface of the leaflets (Fig. 359A).
They are dichotomously 2 to 10 times branched at acute angles (Fig. 359C), ultimate branchlets acute and more or less reflexed. The sporangia are nearly oval (Fig. 359D) but narrowed a little below, greenish-yellow to pale-violet in mass and 22 to 27µ by 15 to 19µ in diameter. They are very delicate structures and germinate by a germ tube in presence of moisture.
The sporangia are blown about freely by the wind and spread the disease rapidly from plant to plant.
They germinate directly by germ tube behaving like conidia. In the old withered infected leaves oospores are found in the host tissues singly within a thin-walled oogonium, which soon disappears. The oospores are roundish, light-brown having a thick epispore marked by a large, raised reticulation.
They measure 26 to 43µ, in diameter. The oospores germinate by a germ tube in the season following that in which they ‘were formed. They can stand unfavourable conditions, like desiccation.
Disease Cycle of Downy Mildew of Pea:
Cool humid condition is very favourable for the development of the disease, while the disease incidence is slowed down with warm condition. This is due to the fact that the sporangia which behave as conidia spread the disease, but cannot stand desiccation.
The disease may be described as:
(i) The primary cycle, when infection starts in the leaflets and stipules from the inoculum produced by the germination of oospores of previous year; and
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(ii) The secondary cycle, when the pods become infected from the sporangia produced by the primary cycle. The discoloured patches on the leaflets and stipules denote successful infection. Host penetration is through stomata by the germ tube produced by sporangial germination.
The sporangia are light and are developed profusely. They spread the disease very rapidly. Whereas, the oospores are responsible for carrying the infection from year to year by tiding-over unfavourable conditions.
Disease cycle of Downy mildew of pea is presented in Figure 360.
Control of Downy Mildew of Pea:
Since the causal organism survives from season to season in the form of oospores in the plant debris, the destruction of previous year’s plant debris and following crop rotation of two or three years are very effective control measures. Spraying and dusting pea plants with fungicides are effective in limiting the spread of the disease.
