502 DISEASES OF CULTIVATED PLANTS 



In potato leaf-curl, as here understood, the leaves curl and 

 the stem droops before there is any external evidence of the 

 cause of injury. At a later stage, however, the stem and 

 leaves bear numerous blackish-olive, minutely velvety patches 

 of various form and size. The patches consist of dense 

 masses of the dark-coloured conidia of Macrosporium solani 

 (Cooke), the cause of the disease. As soon as the leaves 

 begin to curl, a microscopic section of the stem reveals 

 the presence of a dense mass of mycelium, which plugs the 

 water-conducting parts, hence the leaves curl and the stem 

 collapses owing to lack of water. 



When a potato plant is badly infected, the mycelium passes 

 from the haulm into the underground branches and young 

 tubers, and I have explained in detail elsewhere a series of 

 experiments proving that the mycelium present in a tuber 

 used for ' seed ' perpetuates the disease, which in turn again 

 infects the young tubers. By means of such hibernating 

 mycelium the disease is transmitted from generation to 

 generation without the intervention of spores, or without ever 

 leaving the host. This is exactly similar to what takes place 

 in the case of Phytophthora infestans. 



If the disease appears early in the season the crop is much 

 reduced, many roots not producing more than a few very small 

 tubers, some none at all. The conidia, so far as I have been 

 able to ascertain, lose their power of germination, after a 

 period of about three months, but, as a rule, numerous 

 chlamydospores are produced in diseased leaves and haulms, 

 and these bodies germinate readily the season following their 

 formation, hence if such are present in the soil, there is every 

 probability that potatoes planted in such land would become 

 diseased. I stated in A Text-Book of Plant Diseases^ p. 323, 

 that the fungus there called Macrosporium tomato (Cooke) 

 was very closely allied to, if not identical with, Macrosporium 

 solani (Cooke). Subsequent experiments have proved this 

 surmise to be correct ; I have repeatedly produced the disease 

 on tomatoes by inoculation with spores produced on a potato 

 plant, and vice versa. The disease on tomatoes is known as 

 'black rot' when it forms black blotches on the fruit, and 

 ' black stripe ' when it forms blackish lines on the stem. The 

 mycelium is dark coloured in the region where beds of 

 conidia are produced at the surface, but becomes colourless 

 and thinner in the deeper tissues of the host. 



Conidia brown, variable in size and form, clavate, oblong, 



