100 



Percent 

 of Season's 

 Ascospores 

 Released 



50 - 



t.c. pink bloom p.f. 

 Mar 25 Apr 14 May 4 May 24 



Date (growth stage) 



Jun 13 



Figure 1 . Venturia ascospore maturity by date in 1990 at Wilbraham, MA, showing the stage of tree development 

 and the point at which significant levels of scab were first present (5%) and the point at which the season ended 

 (95%). 



April 20, and this level probably would create no 

 infections. In this example, the released spores hit 

 10% at pink and 20% at bloom. At this point, the risk 

 of infection becomes important. But, in low inocu- 

 lum orchards, there is little risk of infection up to this 

 point. 



The question of inoculum from wild hosts has not 

 been answered fully. MacHardy and his associated 

 suggest that inoculum from outside the orchard is 

 not important. For purposes of the delayed SI 

 program, this suggestion appears to hold true. 



Based on this scab inoculum theory, and on the 

 SI effect on pre-symptom scab, it seems likely that 

 the first scab fungicides of the season could be 

 applied much later than is usually done. Such a 

 schedule might eliminate one or more early-season 

 applications. 



How would this strategy for using the 

 Sis actually be done in an orchard? 



In 1987, we began the implementation of the 10- 

 day, delayed-SI program in commercial orchards 

 and research plots, using a total of approximately 50 

 acres. Fungicides were withheld until the tight 

 cluster or pink stage of development, regardless of 

 apple scab spore maturity or the number of infection 

 periods which had occurred. Once begun, the appli- 



cations were made as close to 10-day intervals as 

 possible. The SI fungicides were always applied in 

 combination with a protectant fungicide, for reasons 

 described earlier. Rates and intervals were adjusted 

 to account for differences in disease pressure when 

 necessary. In all cases, a minimum of 2 applications, 

 and preferably 3, were made after the first applica- 

 tion, for a total of 3 to 4 Sl/protectant sprays during 

 the primary scab season. When the interval from a 

 major infection period extended to more than 4 days, 

 we recommended a higher SI rate, tightening the in- 

 terval between the first and second post-infection 

 spray to 7 days, or both. In general, the delayed 10- 

 day SI program was successful, and in later years 

 more acres have been grown using the schedule. 



Have the rules changed since the 

 early 10-day delay trials? 



We stress that each season and each grower 

 required slight modifications to fit individual situa- 

 tions. In an attempt to be comprehensive and guide 

 growers in making individual decisions, we devel- 

 oped a flow chart based on phenological development 

 to guide SI applications. However, this chart proved 

 to be confusing to many growers. As a result, we 

 arrived at a simpler set of rules, and used individual 

 consultations to deal with individual situations. 



20 



Fruit Notes, Spring, 1991 



