Chemical and biological control of grape nematodes in the San Joaquin Valley
By Gabriel Torres, UCCE Tulare & Kings Counties
Nematodes are microscopic roundworms. Nematodes that feed on plants are known as plant parasitic nematodes, and their feeding can cause significant yield loss.
Some species, such as the citrus nematode (Tylechulus semipenetrans) feed on a few specific plant species, whereas other species like the root knot nematode (Meloidogyne incognita) can feed on more than 150 different plant species. Several different nematode species can feed on the same plant, at the same time, further increasing the damage they may cause. Among the 22 genera of plant-parasitic nematodes, 10 have been reported to feed on grapevines.
There are two complementary strategies for nematode management: pre-planting and post-planting management. Pre-planting managements includes soil fumigation, anaerobic soil disinfestation (ASD) and rootstock selection. Post planting management includes both organic and conventional control methods. Cultural practices, such as preventing infested soil from moving from an infested vineyard to a non-infested vineyard by washing equipment, complement both strategies.
Nematode sampling prior to vineyard establishment is important. A sample should be taken for every 5- 10 acres that has had a different cropping system, or different rootstocks in the past, and should also be separately collected for different soil types. Each one of these samples should consist of 1 quart (2 ¼ pounds) of soil collected from up to 24’’ inches in depth. After planting, nematode sampling, following a similar strategy, is recommended every 3 to 5 years. However, if a high population is identified and treated, a follow up sampling at the end of the growing season is recommended to determine the efficacy of the treatment. Table 1 shows the critical levels for different nematode species.
Anaerobic soil disinfestation consists of incorporating organic matter (such as rice hulls, molasses, or mustard meal) into the soil, then saturating the soil, and finally covering the soil with plastic for 4 to 6 weeks. The organic matter acts as a food source for soil microfauna to quickly reproduce. Along with reproducing soil microfauna, the saturated soil and plastic covering limit the amount of oxygen, ultimately resulting in an anaerobic environment within the soil profile. This environment is detrimental for organisms that require oxygen, such as nematodes, fungi, some bacteria, and plants. Fumigants also have a biocidal activity against wide range of organisms, but their use is limited by costs and environmental regulations.
After planting, fumigants and ASD are not viable, and chemical or biological measures are needed to control nematodes. In California, only the chemical active ingredients spirotetramat and imidacloprid are registered for grapes. Azadirachtin, Margosa oil, Myrothecium verrucaria, Purpureocillium lilaciunum, and Quillaja are registered as biological controls. Frequent monitoring and repeated use of these nematicides are required once nematodes populations are established.
When ASD and soil fumigation are not feasible, rootstocks resistant to local nematode populations are required. However, the use of resistant rootstock can also complement those techniques, resulting in a lower risk of develop higher nematode populations, and perhaps in a lower requirement of biological or chemical inputs for their management later during the lifespan of the vineyard.
To improve the efficacy of chemical nematicides they must be applied between budbreak and pre- bloom, or after harvest when the leaves are still active. Sprays during other phenological stages are not effective. It is also important to consider the risk of resistance development, especially when only two active ingredients are available for nematode control, and they are also widely used as insecticides for mealy bugs and other pests, adding to the selection pressure for resistance. Rotation of the active ingredients is advised to prolong the efficacy of these products. Biological nematicides may require more frequent applications to be efficient.
If you have questions, please contact your local farm advisor or your PCA.
Table 1. Nematode population sizes considered to be Low, Medium and High for a range of different nematode types and species at different times of the year. McKenry, 2013