APPLE DISEASES 



89 



are arranged radially about it. These are the fruiting pustules 

 of the causal pathogene. On yellow varieties like the Yellow 

 Bellflower and Tolman Sweet the spots are at first almost a 

 carmine red. Later they turn brown in color. This change in 

 color may occur while the apple still hangs on the tree, particu- 

 larly if the weather is wet for a few weeks prior to harvesting ; 

 or the change may be delayed until the fruit is stored. Spots 

 on affected fruit increase 

 but little in size when 

 placed in cold storage. 



Cause. 



The fungus Phoma pomi 

 produces this apple fruit- 

 spot disease. The patho- 

 gene probably hibernates 

 in the form of sclerotial 

 masses and as peculiar 

 thick-walled cells known 

 as chlamydospores. 

 These structures winter 

 over on fallen fruit. 

 With the advent of the 

 growing season conidia originate from both the sclerotia 

 and the pycnidia on the fallen fruit, and these spores bring 

 about the first infections of the year. Observations show 

 that a larger number of the inoculations take place in July or 

 early August. None occur after the last of August. Entrance 

 into the fruit is gained by way of the stomata. After the 

 spores are deposited on the apple fruit fifty days may elapse 

 before there are any signs of the disease visible to the unaided 

 eye. Owing to this condition of affairs fruits may be inocu- 

 lated before picking but show no evidence of fruit-spot at the 

 time of harvest. In due time, however, the disease develops 

 in storage or in market. On entering the apple-tissue the genn- 



FIG. 24. Apple fruit-spot. 



