The effect of phosphonate rate on control of scab on pecan

Abstract

Scab, caused by the plant pathogenic fungus Venturia effusa, is the major disease of pecan in the southeastern USA. In widely grown susceptible cultivars, frequent application of fungicides is the only option to control the disease. Unfortunately resistance to some fungicides has developed so management of the chemistries available is critical for the industry. Understanding how rate affects disease control is important to ensure that control is maximized. We compared the effect of different rates of phosphonate products in reducing scab in a total of six field experiments from 2015 to 2018 at two locations in Georgia. Cultivar Desirable was used in four experiments, and cv. Wichita was included in 2 years at one location. Depending on experiment, four rates of ProPhyt® (2.3, 3.5, 5.3 and 7.0 L/Ha), three rates of Rampart® (2.3, 4.7 and 7.0 L/Ha), or two rates of K-phite® 7LP (2.3 and 7.0 L/Ha) were tested. Also depending on experiment, 5 to 10 sprays of phosphonate product were applied. Incidence of scab was invariably high on fruit from the non-treated, control trees and most often high on treated trees. However, treatment with the higher rates of phosphonate product most often significantly or numerically reduced severity of scab compared to the lower rates of the products applied on foliage and fruit. The label-stated weight of the active ingredient (phosphonate) per unit volume is similar in these products and they reduced the severity of scab similarly compared to the control and to each other at comparable rates. We found that applying the higher rates we tested (3.5–7.0 L/Ha) most often controlled scab better when compared to the lowest rate (2.3 L/Ha). In 2015 fruit weight was increased where phosphonate application resulted in reduced disease, but in 2016 the effect was unclear due to a late and mild epidemic of scab likely having limited impact on the fruit size. Regardless of epidemic intensity, higher rates of phosphonate provided more efficacious control. Phosphonate products are a vital chemistry to manage scab for the pecan industry in the southeastern USA and our results demonstrate that higher rates contribute to more efficacious control of this yield-limiting disease.