Toxicity of Fungicides to Apple Pollen 



Daniel R. Cooley 



Department of Plant Pathology, University of Massachusetts 



Following a heavy bloom, it is particularly 

 frustrating to see poor fruit set; yet, many Mas- 

 sachusetts orchards have seen this problem for 

 the past few years. There are a number of 

 potential reasons for poor set. Cold stress, oil 

 toxicity, dry weather during flower set, nutri- 

 tional deficiency, insecticide damage, and fungi- 

 cide damage all have been implicated. As yet, 

 nothing has stood out as the prime culprit. 



In looking for a reason for these years of poor 

 fruit set, some work done in England came to 

 light. Over several years, researchers (chiefly 

 Ruth Church at Long Ashton Research Station) 

 examined the effects of different fungicides on 

 apple pollen survival. Obviously, if pollen viabil- 

 ity (or its ability to fertilize eggs) decreases, 

 there will be fewer fruit set, regardless of bloom, 

 bee activity, or pollination weather. 



To make a long story short, it appears that 

 captan can drastically decrease pollen viability. 

 The effect can be seen in both laboratory and in 

 field tests. The results presented in Table 1 do 

 not state what the specific effect on fruit set is; 



however, captan was the only material that 

 severely reduced pollen germination in labora- 

 tory and orchard studies. 



In another study, dilute applications of cap- 

 tan were more toxic than concentrated applica- 

 tions (about 5X). Time of application also was 

 important. The closer to pollination that an 

 application was made, the more chance there 

 was that pollination was inhibited. Church 

 showed that applications made within 24 hours 

 of pollination were very detrimental to fruit set. 

 In one test, hand pollinated blossoms were 

 sprayed with different fungicides either before 

 or after pollination. The results (Table 2) indi- 

 cate that if pollination occurred two hours before 

 spraying, pollen did not grow adequately, but if 

 the pollination occurred two hours after spray- 

 ing, pollen tubes developed relatively well. 



More recently, laboratory tests have com- 

 pared sterol-inhibiting (SI) fungicides and cap- 

 tan. Again, captan was highly toxic to pollen. 

 Dodine was also somewhat toxic in several tests. 

 As Table 3 shows, the SI fungicides did not have 



18 



Fruit Notes, Fall, 1991 



