Metalaxyl-M, phosphorous acid and potassium silicate applied as soil drenches show different chestnut seedling performance and protection against Phytophthora root rot

The application as soil drenches of three commercial products containing metalaxyl, phosphorous acid or potassium silicate, were studied as a means of controlling Phytophthora in chestnut (Castanea sativa Mill.) seedlings. In the metalaxyl treatment no plant deaths were recorded, whereas with the phosphorous acid and potassium silicate applications, and in the untreated control, the mortality rate was respectively 33.3, 44.4 and 77.8%. The presence of Phytophthora was detected in plants and soils at the end of the experiment which means that the three products did not eliminate the pathogen, and that they only gave temporary protection to the plant. In the treatments receiving metalaxyl or phosphorous acid, the plants showed a sharp drop in growth compared to the surviving plants of the control. Soil analyses revealed a high increase in exchangeable acidity, a high reduction in pH and a high increase in Mn levels in the soils treated with the products containing metalaxyl or phosphorous acid. In these treatments, elemental tissue analysis and nutrient recovery by plants revealed Mn levels far above the upper limit of the sufficiency range, with the toxicity of Mn being the suspected cause for the strong reduction in plant growth. These results indicate that when applying such products to the soil, their concentration and/or their ability to influence the soil pH should be evaluated and adequate measures of pH adjustment undertaken. In the case of phosphorous acid, its use can be replaced by phosphite salts. K-silicate did not show adverse effects on plant growth but provided less protection against Phytophthora than metalaxyl or phosphorous acid.

Automated summary

Metalaxyl effectively controls Phytophthora in chestnut seedlings, while phosphorous acid and potassium silicate show lower efficacy and toxicity. The products provide temporary protection to plants but may have adverse effects on plant growth due to high levels of manganese in the soil. Adjustment of soil pH should be considered when using such products.