MACROSPORIUM DISEASE 705 



starch and other plastic reserve-materials and a consequent 

 increase in the yield of tubers. 



It is well to point out that in one or two cases where spraying 

 has been continued till a somewhat late period, the plants have 

 been so slow in ripening that the farmer has not been able to 

 dig his crop soon enough to secure an early market. This 

 experience is, however, exceptional and can easily be avoided. 



Ex. 281. — Examine the dead patches on the leaves of potatoes in July, 

 August and September for Phytophthora infestans. It appears as a greyish- 

 white mould round the margins of the dead patches. 



Observe the progress of the diseased spot from day to day, noticing the 

 colour-changes of the leaf. 



Ex. 282. — Place a piece of diseased leaf, with the underside upwards, 

 on a slide and examine the fungus with a low power. 



Ex. 283. — Cut transverse sections of the leaf through the edge of diseased 

 spot on a leaf and mount in water. Note the sparsely branch conidiophores 

 and the mycelium within the leaf-tissues. The conidia break off so easily 

 that few or none will be observable in sections prepared in this manner. 



Ex. 284. — To obtain a luxuriant growth of the Phytophthora place five or 

 six leaves showing the first symptoms of disease one upon another on a plate 

 under a bell-jar. Sprinkle the leaves with water and allow them to remain 

 for twenty-four hours. The fungus may then be seen as a downy film on 

 almost all the leaves. 



Tear off a small portion of the lower epidermis of a leaf where the parasite 

 is abundant and mount in water. Examine with a high power and examine 

 portions of the conidiophores and conidia. Observe the swollen parts of the 

 conidiophore where conidia have dropped off. 



If no conidia are found attached to the conidiophores, tear off a similar piece 

 of epidermis and transfer immediately to absolute alcohol or strong methy- 

 lated spirit. Leave the preparation in this for thirty seconds, and then care- 

 fully transfer successively to 50 and 25 per cent, alcohol, and finally to 

 water. Examine with a |-inch objective. 



(ii) Macrosporium Disease : 'Early Blight.' 



A disease of the potato leaf frequently spoken of as 'Early Blight' 

 is very common in some parts of America and possibly more 

 common in this country than has been imagined. It is caused 



2 V 



