Interactions Between Mycorrhizal Fungi, Soil Fumigation, and Growth of Grapes in California

Abstract

Grapevines from all 18 vineyards sampled throughout the grape-growing regions of California were heavily mycorrhizal from natural infestations. Glomus fasciculatus, Sclerocystis sinuosa, and Glomus macrocarpus were the mycorrhizal species most frequently associated with grapes in the field. At two sites where methyl bromide (MBr) fumigation was used, some grapevines became severely stunted. This stunting was correlated with a lack of mycorrhizal fungi associated with the roots. When 1,3 dichloropropene (DD) was used as the fumigant, vines were not stunted and were also heavily mycorrhizal. Greenhouse trials indicated that grapevines growing in soil fumigated with methyl bromide became stunted but grew normally when inoculated with mycorrhizal fungi or were grown in non-fumigated grape soil. In one vineyard fumigated with methyl bromide, vines receiving mycorrhizal inoculum grew better and yielded 66% more grapes than vines not receiving mycorrhizal inoculum. In a second field trial, all vines became mycorrhizal within 15 months of planting whether they were inoculated with mycorrhizal fungi or not. However, total growth of all vines inoculated with mycorrhizal fungi were still 13% greater than non-inoculated vines after 19 months. It appears, under certain conditions, that field fumigation with methyl bromide may severely stunt growth of grape vines. These conditions are: 1) effective soil fumigation which destroys the majority of soil-borne mycorrhizal propagules; 2) slow reinvasion of the fumigated soil by mycorrhizal fungi; 3) soil which is deficient in at least one essential nutrient such as phosphorus, zinc, or copper; and 4) planting stock which is non-mycorrhizal.