Control of Phytophthora Rot with Metalaxyl in Established Asparagus

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Abstract
Field plots of asparagus (Asparagus officinalis) established in Yolo loam soil at Davis, CA [USA] and infested with field soil containing Phytophthora megasperma var. sojae were sprayed with metalaxyl at rates of 0.56, 1.12, or 2.24 kg a.i./ha on either 10 January or 20 February 1983 or at 1.12 kg a.i./ha on both dates. The same treatments were repeated on either 29 November 1983 or 20 February 1984. Treatment with metalaxyl increased yield of marketable spears between 35 and 141% in 1983 and between 17 and 44% in 1984. Results of this trial and a second similar trial on peat soil in the San Joaquin Delta Showed that one application of metalaxyl at 1.12 kg a.i./ha applied 7-14 days before the start of harvest resulted in the most cost-effective control of Phytophthora rot during the very wet 1983 season, but a split application of 1.12 kg a.i./ha applied in late autumn and again in early spring produced the highest yields in 1984. Application of metalaxyl to control Phytophthora rot during the winter months while the plants were dormant indicated that winter infections had relatively little effect on production. However, preharvest applications showed that yield losses caused by Phytophthora were greatest when infection occurred in the spring. A significant amount of spear and/or crown rot occurred below the soil surface, suggesting that the effect of Phytophthora on asparagus production in California has been underestimated in the past, especially in wet years.

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