Downy mildew disease of pea shows widespread occurrence in North America and Europe. In India, it is evident commonly in Gangetic plains though reported from other parts of the country also. It is estimated that downy mildew disease does not cause any significant damage to pea crop.
Symptoms of Downy Mildew Disease:
Symptoms are evident on leaves, stems, and pods. The infection occurs very early in the growing season and the symptoms appear generally at the time third and fourth leaves develop on the young plant. First yellow to brown scattered patches appear on the upper surface of infected leaves and stipules and then greyish-violet downy growth of the pathogen can be seen just opposite on the lower surface.
Infected stems become distorted and show reduced growth resulting in subsequent stunting of the entire plant. Symptoms are first noticed on pods when they are green and flat. They are in the form of pale green, more or less elliptical to irregular, elongated lesions mostly confined along the dorsal sutures of the pods, and, in the later stage, they turn from light to dark brown in colour. Seeds lying corresponding to the symptoms on the pods abort and become very much undersized. However, leaves are abundantly infected in comparison to the pods.
Causal Organism of Downy Mildew Disease:
The pathogen is an obligate parasite. Mycelium is coenocytic, hyaline, profusely branched, intercellular sending branched, finger-shaped haustoria inside the host cells. They collect in the air spaces beneath the stomata and give rise to conidiophores emerging out through stomata in clusters. The conidiophores are un-branched for about two-third or more of their length and dichotomously branch (2-10 times) at their tip.
The branches are long, slender and pointed, and are called ‘sterigmata’. A single conidium is produced at the tip of each sterigma. The conidia are oval to elliptical (narrowed a little below) and light yellow in colour. Since the conidiophore is of determinate growth, conidia are produced almost at a time on sterigmata of each conidiophore and they fall off simultaneously. They are short-lived and germinate readily by germ tube.
When the fungus moves towards the end of the growing season, its hyphae move deep into the host tissue interspaces wherein they produce sex organs (antheridia and oogonia) in close proximity. After fertilization via fertilization tube, thick-walled, greenish-yellow, warty oospores are produced at the rate of one in each oogonium.
Downy Mildew Disease Cycle in Pea:
(i) Perennation:
The pathogen perennates in the soil through oospores, which are brought therein along with infected plant debris. The possibilities of carrying pathogen inoculum (mycelium and oospores) by seeds of infected pods and their role in perennation though thought of but have not yet been definitely proved.
(ii) Primary Infection:
The oospores perennating in soil germinate by germ tube at the return of favourable conditions. The germ tube infects underground parts of young seedlings and the pathogen progresses upwardly systemically causing primary infection, which is manifested by symptoms developing on the third and fourth leaves of the young plant. The primary infection results in the production of conidia on infected leaves.
(iii) Secondary Infection:
The conidia produced as a result of primary infection serve as the source of secondary inoculum. They are wind-blown, brought on to the surface of the young pods, and cause secondary infection thereupon by means of each producing germ tube, which penetrates through stomata and gives rise to local lesions.
Pathogen present in all infected plant parts enter in to sexual phase of life producing oospores. The latter are the perennating structures and serve as source of primary infection during the next growing season.
Predisposing Factors:
Cool and moist weather conditions very much favour the disease incidence, while the latter is slowed down considerably if the conditions turn to warm and dry.
Management of Downy Mildew Disease:
1. Since the pathogen perennates in soil in the form of oospores which enjoy long resting period, destruction of previous year’s plant debris and practicing crop rotation for at least two to three years are very effective control measures of the disease.
2. Attempts to control the disease by spraying and dusting pea plants with fungicides have not yet proved successful and economical under field conditions.
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