Influence of rate and frequency of Ridomil plus applications on late blight severity and potato yields in Cameroon

Abstract

The impact of Ridomil Plus® (12% metalaxyl + 60% cuprous oxide) spray rate and frequency on the severity of potato late blight, caused by Phytophthora infestans, was evaluated during 1997 and 1999 in Dschang, Cameroon. A randomised complete block design was used in both experiments. In 1997, plots were sprayed with 0.6, 1.2, 2.4, or 4.8 kg a.i.ha-1 of Ridomil Plus®, while in 1999, they were treated once, twice or thrice with 2.4 kg a.i.ha-1 of the fungicide. In both trials control plots were not sprayed. Late blight intensity was recorded weekly and yields were obtained at harvest. Fungicide applications reduced late blight intensity and consequently increased potato tuber yields. Marketable yields were significantly higher in the resistant variety, Cipira (22.4 - 34.8 t ha-1), than in the susceptible variety, Tezelfeu (4.0 - 12.0 t ha-1). A significant quadratic relationship existed between the rate or number of fungicide applications and tuber yields. Economic analyses indicated that two sprays of the fungicide at 2.4 kg a.i. ha-1 are necessary for a good late blight protection programme in potato. This spray regime resulted to increases in net returns of US$ 1947 to 3327 ha-1. Results also suggest that high rates or frequencies of fungicide applications do not necessarily lead to better economic benefits.



Key Words: Economic gains, fungicide application, Phytophthora infestans, Solanum tuberosum


(African Crop Science Journal 2001 9(1): 235-244)