In cool moist weather the sporangia function as zoosporangia (F 1-3). As the sporangium reaches maturity, the supporting hyphal branch immediately below it swells slightly and continues to grow turning the attached sporangium to the side. The thin-walled, ovoid or lemon-shaped sporangia, each with an apiculate tip, are borne singly at the tips of sporangiophores or their branches. Under suitable conditions of temperature and humidity, the mycelium pushes out hyahne, branched, indeterminate sporangiophores through the stomata of the host leaves (E). It grows and ramifies in the intercellular spaces absorbing nutrition by putting haustoria into the host cells (D). The infected sprouts emerge above ground and produce shoots which contain the mycelium (C). The released zoospores invade the healthy sprouts and bring about infection.Īccording to some, the sexual phase seems to play no role in the life history of the pathogen. The contents of the latter divide to form zoospores. The germ tube after emergence usually ends in a terminal sporangium. They play a significant role as the source of primary infection.Īt the planting time, the resting oospore germinates. The second view is that the thick-walled resting oospores which are found in abundance in the infected tubers are the important overwintering structures. The activated mycelium invades the healthy sprouts (B). It becomes activated at the time of germination of the diseased seed tubers among the planting stock or waste tubers in dump heaps or infected tubers remaining in the ground after a previous crop. The hyphae are both intercellular and intracellular. The mycelium is aseptate conenocytic, hyaline and branched. The causal organism is Phytophthora infestans (Mont.) De Bary. Under moist conditions hyaline mycelial hyphae and sporangiophores push out through the lenticels and appear on the surface of infected tubers. In cool and dry conditions the progress of the disease is slower and the wet rot phase is generally checked. During storage, the bacteria assist to set in the wet rot phase. These dry rot spots remain firm and extend to about half an inch below the surface. There is brownish discoloration of the skin of those parts of the tubers which lie nearest the surface of the soil. They get separate infections while in the hill. Potato tubers are often infected in the field after the tops have been blighted. The sporangia serve to spread the disease in the growing season. 22.7 E) of the pathogen pushing out through the stomata. It consists of sporangiophores and sporangia (Fig. Under moist conditions they decay and emit a characteristic offensive odour.Įxamination of the lesions on the lower surface of the leaf on a dew morning reveals a delicate growth of the fungus parasite in the form of whitish powdery bloom. The blighted leaves curl and shrivel in dry weather. The disease generally first attacks the leaves, and petioles near the ground and the lesions appear on the lower surface of the leaflets on individual plants and then spreads upwards.įinally, a rapid and general blighting of foliage occurs. Under favourable conditions (low temperature and high humidity) the lesions rapidly increase in size involving the whole surface of the leaf. These appear on the tips and margins of the leaflets, rachis, petiole and stem. The disease makes its appearance as small, dead, brownish to purplish black areas or lesions. The conditioning factor is the favourable environment. It may appear as well at any time during the growth period of the plant. The disease first appears on the tops of the plants generally after the blossoming period but mostly in the month of January. In severe cases of infection there is complete loss of the crop, Infection also results in the decay of tubers in the field and storage. Consequently the tubers remain small and reduced in weight. Severe damage to the foliage shortens the growing season (Fig. The damage caused by the disease is frequently very high.
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