Pech scab, caused by the fungus Fusicladosporium carpophilum, overwinters on twigs produced during the previous season. The disease is spread by wind and rainfall, and infection is strongly influenced by temperature and humidity. If not controlled, it causes cosmetic damage to the skin of the fruit that may result in significant economic losses to the grower.
Most peach varieties are susceptible to scab, but some are affected more severely than others. Fungicide applications beginning at petal fall or shuck split and continuing at regular intervals as cover sprays up until three to four weeks before harvest are critical to control this disease. Early spray applications of chlorothalonil (Bravo, Syngenta Crop Protection) or respiratory inhibitors such as azoxystrobin (Abound, Syngenta Crop Protection) or trifloxystrobin (Flint, Bayer Crop-Science) during the shuck split to shuck off stages are especially recommended if disease pressure is high. Regular cover sprays thereafter with captan (Monterey Ag Resources) or elemental sulfur usually provide sufficient control.
Many growers prefer to use sulfur for scab control because it is inexpensive and effective if disease pressure is not too high. Both scientific observation of others and the testimony of some growers have reported that peach fruit from a chlorothalonil/captan program are better colored at harvest than fruits from a sulfur program. It has also been noted that fruits of the sulfur program may have delayed maturity.
In collaboration with Clemson plant pathologist Dr. Guido Schnabel, we recently completed a four-year study examining various fungicide programs to control peach scab and their impacts on fruit quality. We compared the standard chlorothalonil/captan program with programs utilizing micronized (very fine particles) sulfur exclusively. A more intensive sulfur program (11 applications) was compared with a less intensive sulfur program (six applications). This study compared two popular mid-season cultivars, Contender and Cresthaven.
Following four years of field experimentation, we made the following important observations. Scab pressure varied significantly from season to season in the same orchards, and when scab pressure was low to moderate, six applications of sulfur applied at 13 pounds per acre provided effective scab control. However, when scab pressure was high, 11 sulfur applications performed better than six applications. Although very slight reductions were observed in skin coloration and Brix for fruits from sulfur programs (compared with chlorothalonil/captan), these would not adversely impact commercial pack- out and grade designations.
This column by Dr. Desmond R. Layne, “Maintaining Quality With Sulfur”, appeared in the February 2006 issue of The American Fruit Grower magazine on page 53.