Efficient Use of Sterol-inhibiting 

 Fungicides: Questions and Answers 

 About the Delayedl 0-day Program 



Roberta Spitko 



New England Fruit Consultants, Lake Pleasant, MA 



Daniel Cooley 



Department of Plant Pathology, University of Massachusetts 



Planning apple fungicide schedules 

 in the 90's. 



Even though apple growers have had consider- 

 able success reducing fungicide use, the key to 

 commercially successful disease management re- 

 mains efficient fungicide use. Yet trying to develop 

 a plan for apple fungicide use in the 1991 season is 

 a little like trying to keep an old car running. Just 

 when you have one part fixed, another part breaks 

 down. There are several management "parts" that 

 impact a fungicide program: scab, summer diseases, 

 mites, insects, costs, regulations, and public con- 

 cerns. And of course, good disease management is 

 just a part of the real goal, growing a profitable apple 

 crop. This year, many growers will consider using 

 sterol inhibiting fungicides (specifically Nova™ and 

 Rubigan™). In this article, we give a set of guide- 

 lines for the most efficient and effective use of these 

 fungicides. Unfortunately, there are no blanket 

 recommendations for using sterol inhibitors. Each 

 grower's situation is different, and each year is 

 different. The best recommendation we can give to 

 all growers is to study the pluses and minuses of each 

 option. Then when you need advice, ask for it. 



Why do people call these fungicides Si's? 



The sterol inhibiting (SI) fungicides are a new 

 type of chemical, and they act in a much different 

 way than the standard protectant fungicides. Sis 

 are a valuable new asset, but they bring with them 

 a new set of management problems. 



Actually, there are several names for this type of 

 fungicide: demethylation inhibitors (DMIs), ergos- 

 terol biosynthesis inhibitors (EBIs), or sterol inhib- 

 itors (Sis). While the other terms indicate more 

 precisely what these fungicides do, the term SI is 

 most commonly used, and we use it here. Specifi- 



cally, in this article we mean the two apple scab 

 fungicides, Nova (myclobutanil) and Rubigan (fe- 

 narimol). Funginex™ (triforine) and Bayleton™ 

 (triadimefon) also are Sis registered for apples, but 

 Funginex is a less effective scab fungicide than Nova 

 and Rubigan, and Bayleton does not have any sig- 

 nificant effect on scab. 



Can I use these fungicides for 96 hrs 

 of kickback, the way I used captan to 

 get 18 hrs kickback? 



One of the most remarkable features of the new 

 SI fungicides is long-term curative or post-infection 

 effectiveness. When the Sis were first introduced, 

 apple growers were already familiar with the 18 to 

 24 hrs of "kickback" activity given by protectants 

 such as captan and the EBDCs. In addition, they 

 were familiar with the eradication properties of 

 Benlate™ (benomyl), Topsin-M™ (thiophanate- 

 methyl), or Cyprex™ (dodine). Yet the Sis were 

 promising 3 and 4 days of post-infection activity. 

 Unfortunately, researchers, manufacturers, and 

 growers jumped to the conclusion that this type of 

 post-infection activity was essentially the same as 

 "kickback," i.e. when an SI was applied within the 

 prescribed 96 hrs from the start of an infection, that 

 application would always stop the targeted infec- 

 tion. This assumption was rudely contradicted by an 

 event in Michigan. Rubigan was used in a single ap- 

 plication very early in the season to control a serious 

 infection period. A long dry period followed, during 

 which no fungicide was applied. After about a 

 month, growers discovered scab lesions in their or- 

 chards. It became evident that while a single post- 

 infection application will usually eradicate an infec- 

 tion, there are circumstances when it will stop it only 

 temporarily. At least two post-infection SI applica- 



Fruit Notes, Spring, 1991 



17 



