Once Verticillium wilt gets into a field, the potential is always there to infect a host crop, but San Joaquin Valley tomato growers may want to consider rotations of broccoli or mustard for some help in managing the disease.

Mike Davis and others in the Plant Pathology Department at the University of California, Davis, say that broccoli and mustard residues have compounds which do not completely knock out the disease but enhance soil microorganisms that reduce it.

Davis reported on the latest developments in research on Verticillium and Fusarium wilts in tomatoes during a recent vegetable crops gathering at the West Side Research and Extension Center at Five Points.

The thrust of the studies has been to identify races of the two pathogens so plant breeders can develop resistance to incorporate in improved varieties. In the case of Verticillium, Davis said, “we need to know if it is one population or several different strains.”

The two fungi are harmless to wild plants. But after years of cultivated crops they evolved changes that allow them to move into the vascular systems of susceptible crops. After spores mature in residue of the plants, they move through soil or water to infect new plants.

Verticillium cross-infects many species, ranging from lettuce to eggplant and alfalfa to strawberry. Fusarium, with its three clonal races, on the other hand, infects from lettuce to lettuce, or cotton to cotton, although it sustains itself on any host.

However, both pathogens retain their original ability to live on the outer surfaces of crops, survive in soils indefinitely, and can be seedborne, Davis noted.

Verticillium of lettuce, pathologists have learned, has two races. Race 1 is genetically similar that found on strawberry and artichoke. Race 2 is similar to that found on pepper. Both have existed since Verticillium was first found on lettuce in the Salinas Valley in 1995.

Verticillium of tomato also has two races, with plant resistance available for Race 1, but none available for Race 2, which is the focus of current investigations to find out where it originated. Davis said it is known to have four main groups, and researchers are taking these into account to guide breeding programs.

Control of Race 2 at the moment is limited to fumigation with metam sodium, but he added, rotation to broccoli with tomatoes can help. “In general, it’s good to increase microbial activity in the soil. The more of these parasites of the spores of Verticillium there are in the soil, the better.”

Earlier research in lettuce in the Salinas Valley revealed that broccoli residues incorporated into the soil promote these tiny organisms. Broccoli plants are not affected by the pathogen, and their residue causes a 100-fold increase in bacteria and a 1,000-fold increase in actinomycetes, Davis said.

Mustard cover crops are also antagonistic to Verticillium, not because they kill it directly but because of the soil microorganisms it encourages.

With either broccoli or mustard, there is a long-term benefit to improving the microflora of the soil, Davis said.

In a discussion on fertigation in tomatoes, Tim Hartz, vegetable crops specialist at UC, Davis, made distinctions in tissue testing between petiole sampling and whole-leaf sampling.

Values of petiole sampling for NO3-N, PO4-P, and K, he said, can change rapidly over a few days, can be affected by weather conditions, and are useful as a spot-check on the fertility plan.

“However,” Hartz added, “they should not drive your program and they are not useful after full bloom.”

He prefers instead to do whole-leaf sampling for total N, P, and K because it is a better measure of overall crop nutrient status, it changes more slowly than petiole and projects farther into the future, and it can provide useful information at any crop stage.

As a guide, Hartz gave the following tissue sufficiency standards for the two methods:

For whole-leaf sampling, percentage of N should be 4.0 at early flower, 3.5 at full bloom, and 2.7 at first red fruit; percentage of P should be 0.32 at early flower, 0.25 at full bloom, and 0.23 at first red fruit; and percentage of K should be 2.2 at early flower, 1.6 at full bloom, and 0.8 at first red fruit.

For petiole sampling, parts per million NO3-N should be 8,000 at early flower and 3,000 at full bloom; PO4-P at 2,500 at early flower and 2,000 at full bloom; and percentage of K should be 4.5 at early flower and 3.0 at full bloom.

Also of interest to tomato growers was the report from Brenna Aegerter, San Joaquin County farm advisor, on the third-year evaluation in commercial fields during 2008 of a forecasting model for powdery mildew.

Aegerter said while the model may have some value for growers and PCAs, it should not be relied upon as the sole source of information on mildew risk.

The observations, funded by the California Tomato Research Institute, showed at 11 of 14 locations where the disease appeared the model gave a similar level of control as the calendar method (applications at 14-day intervals) of scheduling fungicide treatments. At the other three locations the calendar method gave better control.

Over the three years and 16 trials in the Stockton area, the calendar method averaged 3.5 sprays per season, while the model averaged 2.3 sprays.

The model was developed by farm advisors in the mid-1990s when the disease was less severe. Since the 2007 season, Aegerter pointed out, the pathogen has been sporulating more profusely, developing more rapidly, and becoming more difficult to control. That may be cause for more aggressive attempts to control the disease than the model recommends.

Among the challenges in implementing the model are:

• It assumes presence of inoculum and uniformly susceptible varieties.

• The cost of in-field weather stations, maintenance, and data quality control.

• The sensitivity of the model to small differences in data (both real microclimate differences and errors).

Aegerter recommended growers maximize the fungicides now available for powdery mildew, giving attention to as short an interval as economical and ensuring good penetration of the crop canopy.

“We don’t have that many materials of different classes, but remember to rotate them by classes for resistance management. We know powdery mildew has a very long latent period of up to 21 days from the time a plant becomes infected to when we see symptoms,” she said.

When to treat is an individual decision, but, she added, since the pathogen has become more aggressive, it would be wise to treat earlier than in the past.