I I r 



500 1000 1500 2000 



Accumulated Degree Days Base 0°C from Green Tip 



2500 



Figure 2. Percent of the season's thyriothecia matxired in five Massachusetts 

 orchard sites during 1997, 1998, and 1999 as a function of the degree days (base 

 0''C) accumulated from green tip for the apple cultivar Mcintosh. 



degree days. The mean of these observations was 106 

 degree days, not significantly different from the 125 

 degree days observed in the laboratory. So, it appears 

 that both high humidity and accumulation heat are 

 required to get the maturation process started. 



Surprisingly, humidity did nOt seem to play a role 

 in spore development once it started. When we looked 

 at high humidity degree days throughout the whole 

 maturation process, from green tip to the end of spore 

 development, then they were poorer predictors of what 

 percentage of thyriothecia would mature than using 

 total degree days. 



Would a second generation of thyriothecia develop 

 ascospores in the same growing season? Thyriothecia 

 start to form on blackben-y in mid to late summer and 

 continue to form until late fall. The rate at which the 

 number of thyriothecia increased depended on location 

 (Figure 4). At two locations in the eastern Berkshires, 

 thyriothecia counts dropped slightly in September and 

 October, while counts increased linearly at two 

 locations in south-central Massachusetts. The increase 



in the number of 

 thyriothecia in the 

 southwest was more rapid 

 than that in the northwest. 

 Examinations of these 

 thyriothecia from July 

 through November never 

 found any that matured and 

 produced new ascospores. 

 We feel that is 

 because thyriothecia need 

 to go through a winter, 

 some sort of chilling 

 dormancy perhaps, before 

 they can produce 

 ascospores. When we 

 collected thyriothecia over 

 the winter, they differed in 

 their potential to mature, 

 depending on the date they 

 were collected and on the 

 site where they grew. 

 Thyriothecia collected 

 from three sites in 

 Massachusetts during 

 December 1996 failed to 

 develop after one week of 

 incubation in high humidity 

 at 70° (Figure 3). Of thyriothecia collected in January 

 1 997, 4 to 1 3% developed mature asci after incubation, 

 and the percent that matured in subsequent months 

 generally increased through April, when sampling 

 ended, hi other words, with more exposure to winter 

 temperatures, the fungus is increasingly ready to 

 produce spores when it gets warm. 



Therefore, it appears S. poini has a disease cycle 

 similar to that of V. iimequalis, in that ascospores get 

 ready to grow during the winter and early spring, and 

 then are matured and released when environmental 

 conditions are favorable in the growing season. There 

 is little evidence that conidia or mycelia serve as 

 primary inoculum. While thyriothecia have been found 

 on apple Uvigs and fallen fruit in orchards, it is unlikely 

 that these serve as important sources of primary 

 inoculum in commercial orchards where fungicides are 

 commonly used. 



Like V. inaequalis, S. pomi produces the season's 

 generation of ascospores over a discrete period 

 generally corresponding to phenological development 



8 



Fruit Notes, Volume 69, Summer, 2004 



