Should I be concerned about scab 

 becoming resistant to the Sis? 



Originally, plant pathologists thought that scab 

 would become resistant to the Sis, leading to si- 

 tuations like those seen with Benlate and Cyprex. 

 Cases of scab resistance in the field have been 

 observed in Europe and Canada. To reduce the 

 potential for the development of resistance to scab is 

 one reason why we have recommended using the Sis 

 with a contact fungicide. However, the thinking 

 regarding SI resistance has changed over the past 

 year. First, places where resistance has developed 

 have a history of multiple SI applications for the 

 entire season over a number of years. Second, hot 

 temperatures seem to trigger the development of 

 resistance in scab. In a wild, untreated scab popula- 

 tion, many more resistant isolates are present late in 

 the season than early in the season. Third, the 

 mixing of an SI and a broad-spectrum protectant 

 may not be any more effective in curtailing the 

 development of resistance than the practice of using 

 a limited number of Sis and then switching to the 

 broad-spectrum fungicides. Considering these fac- 

 tors, it may be advisable to use Sis alone during 

 primary season, for 3 to 5 applications, and use a 

 captan program during the summer. Given the al- 

 ternatives and the relatively low risk of resistance 

 developing, the Sis used at a moderate rate (9 oz/A 

 for Rubigan and 4.5 oz/A for Nova) are a good choice 

 and will manage scab well. In general, primary- 

 season use of higher rates of the Sis alone for no more 

 than 2 applications should present no more resis- 

 tance pressure than using a marginally effective 

 protectant with the SI. 



Is mixing an SI with a protectant 

 still a good idea? 



Havingjust read that mixing is not necessary for 

 resistance management, do not forget all the other 

 good reasons for using a protectant. We recommend 

 a protectant, particularly captan, be used with the SI 

 as a deterrent to fruit scab. The protectant will also 

 ensure good fungicidal activity over 8- to 10-day 

 intervals. Protectant fungicides boost the weak 

 protectant ability of an SI to about 5 to 7 days. 

 Combined with the 4-day post-infection activity of 

 the SI, growers can use 10-day schedules confi- 

 dently. Insecticide and fungicide applications can be 

 combined more easily, saving trips through the or- 

 chard. In the past, if growers chose to use Sis, they 

 generally used the 10-day approach in New England 



and combined an SI with mancozeb. However, 

 without the EBDCs, we have had to reevaluate the 

 combination approach. 



If I should not cut rates, and can 

 only save lor2 applications on a 

 10-day schedule, is there some other 

 way to save applications? 



It was clear that growers might use 1 or 2 fewer 

 fungicide applications with the Sis on a 10-day 

 schedule, compared to using other fungicides on a 7- 

 day schedule; however, SI programs remained rela- 

 tively expensive. Recently, we have made further 

 efforts to reduce the number of SI applications 

 needed in a season. We did this by delaying the 

 initial fungicide application made in a season. There 

 were two basic reasons behind the development of 

 this approach. (1) Long-term suppression of scab 

 symptoms. As research on the Sis continued it 

 became apparent that as long as SI applications were 

 made before actual symptoms from an infection 

 appeared, there was a good chance that the infection 

 could be eradicated. While it was best to stay within 

 the 4-day post-infection period, it was possible to go 

 well beyond that time-frame and still get control, as 

 long as the fungus had not broken out and started to 

 produce spores. Because of cool temperatures in 

 early spring, several weeks may be required before 

 lesions from a particular infection period will erupt 

 through the leaf surface. This provides a wide 

 window during which SI fungicides may be applied 

 to arrest developing infections. (2) The inoculum 

 dose factor. At the same time, William MacHardy, 

 David Rosenberger, and David Gadoury proposed 

 that early-season scab inoculum in a commercial 

 orchard may be so low as to be insignificant. Since 

 only a small proportion of the total scab inoculum for 

 a season is mature at the beginning of the year, and 

 there is very little if any scab in a commercial 

 orchard, there should be virtually no inoculum in 

 most commercial orchards at the beginning of the 

 scab season. As shown in Figure 1, it takes a few 

 weeks from the time the first few mature spores are 

 visible, or from green tip, to reach a point where 5% 

 or more of the spores have matured and been dis- 

 charged. Suppose there were 10,000 ascospores per 

 square yard in an orchard. As of April 20, five 

 hundred would have been released, and probably 

 created problems. But suppose only 10 ascospores 

 per square yard were available. Then only 1/2 a 

 spore (on average) would have been released by 



Fruit Notes, Spring, 1991 



19 



