Fungous diseases of plants . hat in-fection may result through the growing twigs. Biting or piercinginsects are doubtless of much importance in spreading the diseasein this way. Injuries and sometimes, perhaps, even water poresmay be the seats of infection. In general, however, it is certainlytrue that the presence of germs upon the surface of healthy tissueswould not result in the production of disease in those parts. The bacillus winters over, under favorable conditions, in rela-tively few affected branches, under conditions where moistureis sufficient and protection from drying out adequate
Fungous diseases of plants . hat in-fection may result through the growing twigs. Biting or piercinginsects are doubtless of much importance in spreading the diseasein this way. Injuries and sometimes, perhaps, even water poresmay be the seats of infection. In general, however, it is certainlytrue that the presence of germs upon the surface of healthy tissueswould not result in the production of disease in those parts. The bacillus winters over, under favorable conditions, in rela-tively few affected branches, under conditions where moistureis sufficient and protection from drying out adequate. It is fromsuch wintered-over areas as centers that the disease is spread to 126 FUNGOUS DISEASES OF PLANTS the blossoms the following spring. With the return of growingconditions, fermentation may be set up and beads of the gummyexudation produced. Since the beads contain countless quantitiesof the bacillus, insects readily spread it to some blossoms; thenceit is promptly carried by bees to greater distances. The P^iG. 33. Pear Fruit infested with the Blight Organism; BeadsEXUDED IN Moist Chamber. (Photograph by H. H. Whetzel) however, is not very resistant to conditions. It is killed by verybrief exposure to sunlight and by a period of drying. This latter,however, seems remarkable, in view of the general experience thatno amount of cold can act unfavorably upon this organism. It ispossible, however, that the effect of cold in the absence of mois-ture may be as disastrous as drying out. The characteristics of this organism according to Whetzel areas follows :
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