Tomato Diseases 187 



Late blight is a disease of frequent occurrence on 

 greenhouse tomatoes. 



Symptoms, Affected plants appear as though 

 killed by frost. The disease first shows itself as 

 small blackened areas on the leaves, stems, and 

 fruits. These rapidly increase in size and cause the 

 premature death of the affected host. Fruits which 

 may not show signs of disease will develop the 

 trouble in transit if coming from infected houses. 



The Organism. The mycelium of the fungus is 

 hyaline, non-septate. As shown by Melhus* and 

 others, the mycelium may be carried from year to 

 year within the infected tubers. In fact this is but 

 one way by which late blight is distributed. 

 Through the stomata of the infected leaf emerge the 

 slender conidiophores bearing the ovoid conidia. 

 According to Melhus the conidia of Phytophthora 

 infestans may germinate either directly by a germ 

 tube or by the production of zoospores as in Pythium. 

 The best germination occurs at the optimum tem- 

 perature, which lies between lo and 13 degrees C. 

 (50-57 degrees F.). The conidia may be killed by 

 exposure for six to twenty-four hours to dry atmos- 

 pheric conditions such as exist in an ordinary room. 

 Frost which kills the top of the plants will also kill 

 the conidia of Phytophthora. Light does not hinder 

 germination and therefore has no inhibiting effect 

 on infection. Investigation fails to show that Fhy- 

 tophthora infestans produces sexual spores or 00- 



* Melhus, I. E., U. S. Dept. of Agr. Jour. Agr. Research, 5 : 59-65, 

 1915. 



