Calcium Sulfate Soil Amendment Reduces Incidence of Phytophthora Root Rot in Raspberry

Abstract

Phytophthora is a debilitating root disease of many crops, and limits raspberry production in much of the world. Two fungicides are labeled for use against Phytophthora on raspberries in North America, but they are not effective when disease pressure is severe. We were interested in the effect of preplant soil amendments such as composts, fertilizers, gypsum and limestone on the incidence of Phytophthora in red raspberries. Various amendments were incorporated simultaneously at two sites prior to planting raspberries. Both sites had identical soil, but one had a known history of P. fragariae var. rubi and the other had no history of raspberry production. Differences in survival and plant growth of `Heritage' among fertilizer (ammonium nitrate, phosphorus acid), compost, gypsum and lime (calcitic or dolomitic) treatments were small in the uninfested site, whereas differences were large in the infested site. Gypsum (CaSO4) treated plots had higher yields and greater cane diameter, density, and height compared to control plots after 2 years. Plants in compost amended plots exhibited the most severe disease symptoms; liming had little effect. In a second field experiment, various sources of calcium and sulfate were compared in an infested site. As before, plants in calcium sulfate-amended plots performed better than plants in control plots or in plots amended with sulfur or potassium sulfate. In a greenhouse experiment using infested soil and artificial flooding, calcium sulfate also reduced disease incidence in `Titan' compared to unamended soil. Data from these three experiments suggest that gypsum could be used in an integrated approach to Phytophthora management in raspberries, and perhaps other crops as well.