TOMATO LEAF MOLD AS INFLUENCED BY ENVIRONMENT 



By E. F. Guba, Research Professor of Botany 



INTRODUCTION 



Tomato leaf mold, caused by the fungus Cladosporium fulvum Cke., has been 

 the subject of numerous investigations, particularly in England, Germany, and 

 the United States. For several years, the disease has been studied in Massachu- 

 setts in an effort to further knowledge of its control in the greenhouse. More 

 recently, the work has been confined entirely to the development of suitable 

 resistant varieties. It is the purpose of this paper to offer the results of studies 

 dealing with the relation of environmental factors to the development and parasit- 

 ism of the causal fungus, together with a comprehensive review of the pertinent 

 literature. In view of the investigations now being pursued in different countries 

 on the genetics of the tomato in relation to the parasite, the publication of these 

 studies would appear opportune. 



THE DISEASE 



The reduction in yield and quality of tomatoes under glass as a result of the 

 destruction of the foliage by the leaf mold fungus is familiar to every grower. 

 The planting started late in the winter, which is designated the spring crop, 

 usually shows signs of disease in the month of May. The disease gradually becomes 

 epidemic as summer weather approaches. The second planting, started in July 

 and August and designated the fall crop, is very seriously affected from the 

 beginning, and usually by November, especially if the heating has been poorly 

 managed, the foliage is destroyed except for a tuft of uninfected leaves on the 

 tops of the trellised plants. The loss of foliage retards and checks the growth of 

 fruit and reduces the volume and quality of the crop. This loss is much greater 

 in the fall than in the spring. Usually, because of late incidence of the disease 

 in the spring season of cropping, no real or apparent loss in yield is admitted. 



The leaf mold fungus also causes a black stem-end rot of tomatoes. The rot 

 results from the invasion of the fruit at the stem end by the mycelium of the 

 fungus in consequence of spore infection of the blossom (9, 10, 13). When the 

 blossoms and subsequently the ovaries are infected, the young fruits fail to develop 

 and fall from the plants. The invasion of the more mature fruits with subsequent 

 stem-end decay also makes possible the infection of the seed and the transmission 

 of the fungus with the seed. This black stem-end rot is rare in Massachusetts 

 greenhouses. 



Seasonal climatic variations appear to govern the prevalence of the disease in 

 greenhouses. In England the disease is most severe from May to August and 

 only of slight importance from November to March (27, 31). In New Zealand 

 there is no leaf mold for the first three to four months of growing, but with the 

 occurrence of warm, wet weather in October and November, the disease may 

 considerably damage the crop (6). In Massachusetts the disease is not restricted 

 to greenhouse tomatoes. Its occurrence has been noted in epidemic form in out- 

 door plantings adjacent to greenhouses cropped to tomatoes and in stagnant 

 areas associated particularly with uneven topography of the land and poor 

 atmospheric drainage. The disease is of frequent occurrence in hotbed and cold- 



