2005 vegetable diseases again in 2006?

Spinach growers in coastal California counties fought more downy mildew and damping-off diseases in 2005 and should watch for them in 2006, according to a Monterey County plant pathology farm advisor.

Steve Koike, speaking at a plant disease seminar in Salinas, said the spinach industry is under tremendous pressure to continue output of top quality and high volume.

“That’s why downy mildew, the major foliar disease of spinach, and damping off, caused by a complex of fungi, are our two main concerns for the crop,” he said.

The 2005 season also brought other disease developments in coastal vegetables that need to be monitored in 2006.

Koike said virtually all the samples of downy mildew recovered on spinach in the coastal region in 2005 were identified as race 10, although a few samples were of race 6. Race 10 is also the predominant race in spinach in the desert, where race 6 has not been detected.

The main control has been the pair of resistant varieties, Lazio and Amelia, but Koike said seed companies hope to make more resistant varieties available.

During the winter of 2005, examples of race 10 downy mildew, low in concentration and incidence, showed up in a field of the Lazio variety. That raised suspicions that those finds signaled the appearance of an eleventh race, but all samples have been identified as race 10.

Due to their sparse distribution, Koike said, these cases were likely off-type plants that did not carry the true Lazio resistance.

Defense materials

Other available defenses include materials such as the fungicide Aliette and the plant supplement Blockade. Meanwhile, spinach data bases are being developed for new fungicides used on lettuce and manufacturers are seeking registrations for spinach for their products.

“With all the new acreage of spinach going in, we are seeing more soilborne problems like damping off, which is caused by a complex of Pythium, Fusarium, and Rhizoctonia,” he said.

“We all know it’s not a good idea to plant back-to-back spinach or to plant two or three crops of spinach in a calendar year, but that is being done. Ranches that have heavy spinach cropping sequences see these diseases more frequently.”

Pythium strikes first, immediately after emergence or even pre-emergence, and then the others tend to occur later after spinach is in the four-leaf stage. The Fusarium that goes to spinach is not the same as that on lettuce or celery.

The Rhizoctonia of spinach is the same R. solani species that attacks cauliflower and other crops, so care needs to be taken with rotations.

Although Pythium can be managed to an extent with Ridomil applied for downy mildew, Koike said, rotation is the only practice for management of Fusarium and Rhizoctonia.

Celery diseases

Turning to celery diseases, he said he saw more aster yellows in celery during 2005 than at any time in the past 15 years. A type of bacteria that was earlier thought to be a virus, this pathogen is vectored by leafhoppers and distorts and yellows celery. It also goes to iceberg and romaine lettuce.

Although not a serious problem, aster yellows tends to show up in certain fields with regularity, suggesting reservoirs of weed hosts exist between infections. Koike recommended avoiding planting celery and lettuce in fields having a history of the disease.

He said 2005 also brought instances during the spring and summer of another infrequent disease, black rot, on cauliflower, broccoli, and Brussels sprouts. The bacterial, seedborne pathogen causes black, triangular lesions and, at times, black veining on leaves.

In a typical situation, transplants from infected seed are grown under sprinkler irrigation, which splashes infection to other plants. “In all the cases we worked with in 2005, sprinklers were involved,” Koike said. Weeds of the crucifer family are hosts and also contribute to infections.

Generally, if these Xanthomonas campestris bacteria are present in a field alone without a crop-residue host, they do not survive longer than 40 to 60 days after being disked under. But if residue of cauliflower, for example, is present to support them, they can be recovered from the soil more than a year later, so rotation with non-host crops is important.

Another disease, black leg, caused by a Phoma fungus, appeared on crucifer crops in desert areas in 2005, and Koike said although it has not been a problem in the Salinas Valley, it has potential to become one. It, too, is seedborne and can become established with transplants under sprinkler irrigation.

Bacterial leaf spot, a predominantly seedborne disease of lettuce, caused some serious problems in the Salinas Valley in 2005, he said.

“It’s on the seed, but it also is soilborne for short periods of time and that’s how it is persisting in our cropping system. We had outbreaks of it from late July to the end of the season.”

Needs week hosts

Another Xanthomonas campestris pathogen separate from the black rot of crucifers, leaf spot goes to all lettuces. It depends on weed hosts and is worsened by sprinkler irrigation.

Koike’s inoculation trials showed it can carry over in the soil, and he recommended fields infected with it be taken out of lettuce for five months or longer.

Tomato spotted wilt virus, which fluctuates from year to year in coastal lettuce fields, was found again during 2005. A thrips-vectored disease common to peppers, especially in the past two seasons, and tomatoes, its dark-brown, burnlike symptoms in lettuce resemble phytotoxicity. Difficult to diagnose in the field, it and other viruses need lab tests to identify.

The perennial “moving target” for Salinas Valley lettuce growers, downy mildew, has put plant pathologists to work monitoring three of its pathotypes, 6, 7, and 8. Several reports from growers in the desert and Watsonville and elsewhere indicated that these were controlled by Ridomil during 2005.

Koike said tests confirmed Ridomil is indeed effective on samples taken from those areas. “This is counter to what we’ve been saying the past 10 years or so. We can’t say why this switch in sensitivity occurred.”

He said all the lettuce downy mildew strains remained sensitive to Aliette during 2005, although some strains were identified as resistant a couple of years ago.

Lettuce wilt

Another surprise for 2005 was the arrival of Fusarium wilt on iceberg lettuce in southern Monterey County. Long known in the Huron and Yuma areas and other lettuce growing areas of the world, it can be spotted by a general yellowing, wilting, and decline of lettuce plants. It is commonly spread by equipment.

“Look for darkened vascular tissue down the center core of the plant. If you find a plant like this, bring it in to our office and we’ll run a test on it,” he said.

Koike said Fusarium wilt symptoms can resemble Verticillium wilt (not presently in the Salinas Valley), advanced corky root, and ammonia toxicity. Fusarium, he added, is associated with warmer temperatures and would not likely survive in the cooler, northern end of the valley.

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