hoot crops. t93 



Leaf-eating Ladybird Beetle (Epi^chna 28-punctaia). 

 This 28-spotted Ladybird Beetle causes great damage to potatoes, tomatoes, 

 I'unipkins and allied plants, and is illustrated and described under pests and 

 diseases of Cucurbitaceae (page 718). 



Fungus Diseases.* 



Late Blight, or "Irish Blight." 



The potato is liable to be affected with many different fungus diseases, but 

 the most widespread and destructive of all is " Late Blight." This 

 disease is caused by a fungus known as Phytophlhora infestans (Montagne 

 De Bary), which is an active parasite, and can attack all parts of the plant 

 — leaves, stem, and tubers. As a rule it does not attack the leaves until after 

 the blossoming period, hence the name Late Blight. 



Symptoms — Leaves' affected with Late Blight usually become yellowish- 

 green, and develop characteristic irregular dark areas. These frequently 

 appear near the margins and tips and take on a water-soaked, muddy- 

 brown appearance. If the weather remains damp the diseased areas 

 rapidly enlarge and blacken. The leaves become wilted and soon rot, 

 emitting a strong odour. On the under surface, within the advancing 

 edge of the patch, a delicate whitish mould or downy mildew may be 

 seen, especially if the leaf is held in a very slanting position. The destruction 

 spreads to the whole leaf and then to the stem. Finally the whole plant 

 wilts and becomes a crumpled, rotten black mass. The appearance is some- 

 what like that resulting from a very severe frost. If a dry spell or a succes- 

 sion of drying winds should occur after infection, the brown patches may 

 become brittle, crack, and dry up. This condition may mislead the grower, 

 as the appearance is like that caused by Early Blight, due to Alternaria 

 solani, which appears- as dark, irregular spots on the leaves, usually 

 marked by concentric rings. These spots appear earlier in the season 

 and more particularly in drier weather. The leaves may curl up and 

 become brown, and often the spots fall out. Very often the first signs of 

 Late Blight may be seen in the lower leaves on plants, here and there. 

 Although the disease usually becomes serious after the flowering stage, 

 plants may be attacked at any stage of growth. From the time of 

 infection of a healthy leaf until outward visible signs of disease appear 

 a period of only five days is necessary, when the new generation of spores 

 produced will be capable of infecting neighbouring plants. 



With favourable weather conditions an infected leaf may be killed in a 

 day, so that all plants near the original source of infection may be killed 

 within a week. The area of infection continually spreads and a field may be 

 largely infected before the attack is noticed. The fungus may be spreading 

 through many plants for days, living within the tissues without apparent 

 injury to its host, and then suddenly assume a devastating virulence, a 

 condition chiefly controlled by the weather, but also having some relation to 

 the food supply afforded by the host plant. Cool, moist conditions favour 

 the growth of the fungus 



* Compiled by Officers of the Biological Branch. 



