202 



The Philippine Journal of Science 



Fig. 15. 



Phytophthora colocasiae 

 Conidia (X 330). 



conidiophores. The conidia are 

 large, thin-walled, smooth, and 

 colorless and have short, broad 

 papillae (fig. 15). No oospores 

 have been observed. Infection 

 takes place by the conidia, which 

 are scattered chiefly by water. 



The fungus grows readily in 

 pure culture and can be easily 

 isolated by the simple method 

 of plating out diseased portions 

 on potato agar. A downy mass 

 of white mycelium develops on 

 potato agar slopes, and conidia are formed in abundance. Sex- 

 ual spores are produced in pure culture. 



Inoculation experiments, in a damp chamber, produce typical 

 leaf spots in two to three days. 



Control. — Control consists in the growing of disease-resistant 

 varieties. Spraying with Bordeaux mixture is effective. 



Xanthosoma sagittifolium Schott, a heavy-yielding yautia, is 

 not attacked by the Phytophthora and should replace the 

 ordinary gabis. 



CUCUMIS SATIVUS LINN. CUCUMBERS 

 DOWNY MILDEW: PLASMOPARA CUBENSIS (B. ET C.) HUMPHREY 



Symptoms. — Yellow spots are at first produced on leaves. 

 The whole leaf then turns yellow, shrivels, and soon dies. Cen- 

 tral parts of older spots become dead and brittle and are a 

 light brown. The disease starts with the older leaves and ad- 

 vances to the younger ones. Few cucumbers are produced on 

 diseased plants. 



Causal organism. — The typical branched conidiophores are 

 produced singly or in small clusters from the stomata. Conidia 

 are oval and light brown to violet-tinted. 



Control. — Spraying with Bordeaux mixture should be done in 

 severe cases of infection. 



LEAF SPOT: CEKCOSPORA 



Symptoms. — Irregular to angular light greenish leaf spottings 

 are found upon cucumbers. The spotting is not severe, 



CUCURBITA MAXIMA DUCH. CALABAZA, SQUASH 

 DOWNY MILDEW: PLASMOPARA CUBENSIS (B. ET C.) HUMPHRBTif 



Symptoms. — This disease is similar to that discussed under 

 Cucumis sativum Linn. 



