208 DISEASES OF TROPICAL PLANTS CH . 



the fungus known as Phytophthora infestans, De Bary. 

 The slender, filamentous, unicellular mycelium spreads 

 throughout the host plant, finally coming out through 

 the stomata on the under side of the leaf, where it can 

 be seen, by the use of a hand lens, as a delicate white 

 mould. Examination shows this mould to be a branched 

 hypha with terminal conidia. These spores almost 

 immediately give rise to about eight zoospores each, 

 which swim for a short time in the moisture on the 

 surface of the leaf, come to rest, germinate, and penetrate 

 the host plant. They may be carried from plant to 

 plant by the wind, by insects, by the labourers, and 

 farm implements, or they may fall upon the ground, 

 and cause an infection of the tubers. No oospore or 

 resting spore has been discovered for this species, but it 

 is doubtless carried from season to season by tubers. 

 This disease is also known to attack the tomatoes, but 

 is not often destructive (Fig. 25). 1 



Fortunately it can be controlled : first, by the use of 

 clean, healthy seed, and second, by frequently spraying 

 with Bordeaux mixture, which prevents the spread and 

 germination of the spores. The first application of Bor- 

 deaux should be made when the plants are 4-6 inches in 

 height. Other applications should be made in accordance 

 with the conditions of the weather and spread of the 

 disease. The progressive grower will also endeavour to 

 secure varieties which are partially resistant to the disease. 



Early Blight or Leaf Spot. Although this leaf spot 

 is not so destructive as the late blight it has a much 

 wider distribution, and the losses are sometimes said to 

 amount to as much as 50 per cent of the crop. 



In temperate climates it occurs earlier in the season 

 than the late blight. It produces brown spots which 

 are more or less circular, and can be readily distinguished 

 from the late blight by the faint concentric circles which 

 give a 1 characteristic target-board appearance to each spot. 

 The spots frequently unite, forming irregular blotches, 



1 Since writing the above, the oospore has been discovered by Dr. Geo. P. 

 Clinton. 



