Effectiveness of Lime-Sulfur and Phosphorus Acid for Controlling Summer Diseases on Apples

Abstract

Liquid lime sulfur (LLS) was applied to apples at various rates and timings to determine its effectiveness for controlling sooty blotch, flyspeck, and summer fruit rots. Four applications of LLS at either 2 qt or 4 qt controlled flyspeck just as well as four sprays of Topsin M plus Captan (the commercial standard). Four applications of LLS at 1 qt/100 gal were less effective, but applying the low rate six times improved performance to equal that of the standard treatment. Two phosphorous acid products, Phostrol and NutriPhyte, were substituted for captan in several summer sprays and also provided effective control of sooty blotch and flyspeck. Measurements of the spray solution pH and surface residue pH from newly sprayed leaves verified the alkaline nature of LLS sprays and raised the possibility that LLS sprays could cause rapid breakdown of products that are subject to alkaline hydrolysis (e.g. Imidan), thereby compromising effectiveness of those products for controlling apple maggot. No apple maggot damage was observed in test plots, but the incidence of apple maggot damage was low throughout the region in 2006. Based on this trial, organic farmers could adopt LLS sprays during summer to control sooty blotch and flyspeck, but additional work is needed to determine if summer sprays of LLS sprays adversely affect fruit size or productivity of the sprayed trees. The PA products provide interesting alternatives for controlling summer diseases on apples, but considerable additional work is required to determine how these low-risk fungicides can be used to best advantage in apple spray programs.