Tomatoes are hosts to several species of root-knot nematodes (Meloidogyne spp.). Most processing tomato varieties, with some notable exceptions including AB2, and some fresh market tomato varieties possess the Mi gene which confers resistance to several of the species that attack tomatoes.

Tomatoes growing in infested soil that exhibit less than five galls per plant are considered resistant. However, because such varieties have been so intensively used over the years — sometimes planted in consecutive years in the same field — there has been very strong pressure selecting for nematodes which can reproduce on these varieties.

These so-called ‘resistance-breaking’ root-knot nematodes were first found in 1995 and have recently been documented in Yolo and Merced counties by UC Davis nematologist Valerie Williamson. Such strains likely occur in San Joaquin County as well, and would be one possible explanation for the recent occasional problems with nematode damage in resistant tomato varieties.

Root-knot nematodes cause distinctive galls on the roots. The damaged roots have a reduced capacity to supply the vine with water and nutrients. Infected, above-ground plants may be stunted, yellowing, prone to wilting, and respond poorly to fertilizer.

Affected areas usually appear as irregular patches and are frequently associated with lighter-textured soils. Nematodes can also cause damage in heavier soils. The nematodes are active at temperatures above 64 F, but 90 F is optimal for development. If soil temperatures are very high (above 82 F) when the nematode enters the root, then the resistance of the plant may break down, but soil temperatures this high are rare.

Managing fields with nematode problems can be difficult. Most other crops we grow also support reproduction of root-knot nematodes, so rotation out of tomatoes may not be effective.

In fact, even nematode-tolerant alfalfa varieties can support the reproduction of the nematodes, even though they may not be damaged by them. A weed-free fallow is effective, but not likely to be implemented due to the costs of weed control and the losses of keeping land out of production. Pre-plant fumigation or other chemical controls (oxamyl –Vydate) are expensive.

In the future, we will likely be relying on plant breeders to incorporate new sources of resistance into horticulturally acceptable varieties. In the meantime, be aware of which fields may have nematode problems. Check the roots of a few plants in midseason or at harvest, especially if you have poor growth.

Soil samples can be sent to a lab for analysis of nematode populations. If you need help finding a lab, contact me for assistance. Soil samples can be taken anytime prior to planting. If the report from the lab gives an estimate of the total number of root-knot juveniles in the soil, use these figures directly in making management decisions.

If the lab report gives the number of nematodes extracted from the samples, then you must also know the recovery rate. The recovery rate tells you what percentage of the nematodes in the soil is typically extracted; for root-knot juveniles. This number is usually from 10 percent to 30 percent.

To get the total number of nematodes in your soil, divide the lab report number by the recovery rate and multiply by 100. Unfortunately, these standard lab tests cannot discriminate between resistance-breaking and non-resistance-breaking nematodes.

If you have a history of growing nematode-resistant tomato varieties and have a nematode-damaged tomato crop of a resistant variety, we can take plant samples and have the nematodes extracted from the roots and analyzed at UC Davis to determine if they are resistance-breaking nematodes.

However, this is a lengthy process and the results may not be back in time for you to make a planting decision the following spring. Notwithstanding this limitation, such information could be valuable to you in the long run.

To view the tables associated with this report, go online to http://cesanjoaquin.ucdavis.edu/newsletterfiles/Field_Notes_Newsletter17316.pdf.