74 



the disease is present at all, there is a risk of a proportion 

 of the tubers becoming diseased; further, the attack of 

 the fungus on the leaves invariably results in a diminished 

 crop. The first outbreak of the disease in the season is 

 usually dependent upon the weather conditions. In this 

 'country a few days of close, moist weather about the 

 middle of July are often followed by an outbreak of the 

 disease, and, if such weather persists, an epidemic fre- 

 quently results. 



The disease is easily detected by the characteristic spots 

 on the leaves. These often appear near the margin or tip 

 of the leaf and they usually present a dull, water-soaked 

 appearance. As the disease progresses the infected spots 

 increase in size, become darker in colour, and may rapidly 

 involve the whole leaf -blade, extending from the latter 

 to the petiole and even to the stem. Ultimately the leaves 

 hang limp and the whole plant becomes moist and 

 blackened. Such diseased plants emit a peculiar, offensive 

 odour which is very characteristic of the disease. If the 

 attack is a severe one, it is generally found on lifting the 

 tubers that they also are diseased. If in the earliest stages 

 of the disease, the dark spots on the leaves are closely 

 examined on the under surface, a delicate, white, inildew- 

 like growth will be evident round the margin of the dis- 

 coloured spot. This growth follows the advancing 

 margin as the diseased area increases in size. Microscopic 

 examination shows the mildew appearance to be produced 

 by filaments of the fungus Phytophthora, which project 

 from the lower surface of the leaf. The growth as a rule 

 has a somewhat powdery appearance produced by large 

 numbers of spore-like bodies, which readily fall free from 

 the filaments bearing them. Thin ^sections through a 

 diseased spot show that at this stage the threads of fungus 

 run between and within the cells of the leaf. Many of 

 these cells are shrivelled and dead, and the contents 

 having turned brown, they contrast strangely with the 

 taut living cells lined with colourless protoplasm and 

 green granules of chlorophyll. 



In many places branches from the filaments of tho 

 fungus pass out through the air pores or stomata, which 

 are especially abundant on the lower surface of the leaf. 

 These aerial filaments become branched and bear small 

 lemon-shaped bodies, called conidia, on the tips of the 

 branches. The production of such conidia enables the 



