tions are needed to insure that an apple scab infec- 

 tion is stopped. 



This is a key difference between Sis and the 

 other apple fungicides. To use the Sis effectively, it 

 is important to know such differences, and to under- 

 stand a little about the way Sis work. 



Why do people stress slow drying 

 and careful application with the Sis? 



Unlike captan, mancozeb, or the benzimi- 

 dazoles, the Sis are poor protectants. They do not 

 form a barrier on the outside of the leaf. Instead, 

 they act on fungi growing within leaf tissue, killing 

 the fungal cells by disrupting cell wall formation. So 

 the Sis must be absorbed by the leaf tissue to be 

 effective. After absorption, there is little if any 

 fungicide redistribution on the leaf surface. After a 

 few hours, the fungicide is almost completely ab- 

 sorbed by the leaf. Any fungicidal activity then takes 

 place within the leaf. 



As a result, for adequate fungicide action, good 

 absorption is essential. The spray solution needs to 

 be on the leaf as a solution for at least 1 hr and 

 preferably 2 hrs if it is to be absorbed adequately. 

 There are a few conditions which can reduce ade- 

 quate absorption. 



■k Fast drying, as might occur in the heat of a dry 

 day, can decrease SI uptake. 



•k If the spray solution is washed from the leaf 

 soon after application, it will not be absorbed 

 adequately. Applying Sis in a rain, or just 

 before a rain, will not be effective. 



* Thorough coverage during application is 

 more critical with Sis than with other fungi- 

 cides. For one thing, Sis are used at very low 

 rates, compared to the 4 to 6 lbs per acre per 

 application used with materials like man- 

 cozeb or captan. For Nova the total applica- 

 tion for the entire season may be only 1 lb, 

 while for Rubigan it may be 1 qt. Since Sis are 

 not broadly redistributed to uncovered areas 

 of tissue, it is important that tissue be well- 

 covered with the correct concentration during 

 the spray application. 



Getting an effective fungicide dose into the leaf 

 is critical. To work, an SI must cover all the exposed 

 tissue, and be absorbed at a rate which will kill any 

 growing scab fungus. If the rate is too low, it will not 

 kill the fungus. The margin for error with Sis is 

 much smaller than with the protectant fungicides. 

 Cutting rates below the recommended minimum, 



either intentionally or by mistake, will lead to con- 

 trol problems. 



But why should I have to worry 

 more about coverage and rates with 

 the Sis than I did with the fungicides 

 I used to use? 



These days, taking care to put the correct 

 amount of pesticide where you need it is very impor- 

 tant, regardless of whether it is an SI or anything 

 else. However, compared with Sis, the standard 

 protectant fungicides offer superior redistribution, 

 generally affect a broader disease spectrum, and 

 permit growers to make small calibration or applica- 

 tion mistakes without disastrous results. For ex- 

 ample, if there is some small, unknown problem 

 preventing even coverage with a protectant, the 

 next rain will spread out the fungicide. In addition, 

 as many growers know, cutting the label rate of 

 mancozeb by 10%, or even 50%, generally will pro- 

 vide effective scab control. Protectant fungicides are 

 more "forgiving" than the Sis . 



Rate errors with the Sis are much more critical. 

 First of all, when dealing with ounces per acre rather 

 than pounds per acre, it is easier to make a 10%, 20%, 

 or 30% error in weights. Second, our tests and field 

 experience indicate that failures of an SI appear first 

 on the fruit. Dr. David Rosenberger, plant patholo- 

 gist from the New York State Agricultural Experi- 

 ment Station, shows that 1 lb per 100 gal of captan 

 will lead to 4 times as much terminal scab as fruit 

 scab, while Nova at 1 oz per 100 gal will lead to 15 

 times as much fruit scab as terminal scab. So if too 

 low a rate, or bad coverage, lead to scab problems, 

 those problems will often occur where they hurt 

 most: on the fruit. 



Will Sis affect more than just scab? 



Sis must be considered as part of an overall apple 

 disease management program. Generally, they are 

 effective against scab, mildew, and rust. Other 

 diseases, such as frog-eye leaf spot and blossom-end 

 rot, are not controlled by Sis. The Sis are of little or 

 no use on summer diseases such as sooty blotch, 

 flyspeck, black rot, and bitter rot. In order to control 

 these diseases, Sis must be mixed with fungicides, 

 like the protectant mancozeb. However, it is not 

 clear whether or not early-season mancozeb reduces 

 diseases which show up in the summer, and whether 

 or not using Sis alone will cause an increase in the 

 summer diseases. 



18 



Fruit Notes, Spring, 1991 



