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I 



on diseased leaves. In wet weather these spots present a purpHsh or brownish 

 black water soaked appearance, the leaf is killed in one to four days and it falls 

 limp and soon rots. In dry weather the spots are brown, the leaves are not so 

 rapidly killed and they shrivel up soon after death. Infected tubers are charac- 

 terized by the presence of brown or purplish spots on the skin. If the soil is 

 wet and heavy, bacteria will attack the diseased tubers causing a wet rot. In dry 

 light soils, the discolouration does not extend ver}' far into the tuber, the diseased 

 cells die and the spots become slightly sunken. This is the characteristic dry rot. 

 Under favourable conditions the rot may develop and cause considerable loss in 

 stored tubers. 



Late Petato Blight. A downy mildew showing the threads 

 between the cells of the leaf. 



Morphology. — The mycelium differs from that of the other genera of this 

 family in being partly intracellular. The haustoria are small and thread like. 

 The conidiophores emerge singly or in small groups from the stomata on the 

 under side of the leaf. Egg-shaped conidia are cut off from the tips of the 

 conidiophores. The latter continue growing, pushing each conidium to one side 

 as it is formed and producing another at the tip of the new growth. The conidia 

 germinate directly, by sending out a germ tube, or indirectly, by the formation 

 of zoospores. Resting spores are not known to be produced in the field but have 

 been obtained in artificial culture media. 



Infection, (i) Conidia may be carried from leaf to leaf by the wind. 



(2) Conidia will pass from tuber to tuber in the soil, and will infect to a 

 depth of six inches. The mycelium however v/ill not pass from, tuber to tuber 

 through the stem. 



(3) In storage, if the tubers are moist, conidia may be formed which will 

 spread the disease. 



(4) The mycelium may live over in seed potatoes. 



Methods of Control. 



(i) Planting of resistant varieties. 



