Carrot tests sort cavity spot pathogens

In an effort to help carrot growers determine when to apply fungicides for cavity spot, University of California researchers are sorting out ways to quantify soilborne Pythium species that cause the disease.

Michael Davis, plant pathologist at the University of California, Davis, reported on his studies at the recent annual research symposium of the California Fresh Carrot Advisory Board at Bakersfield.

Cavity spot shows as depressed, irregular lesions across mature carrot tap roots, making them unmarketable. These begin as pinpoints that enlarge up to half an inch or larger in diameter as roots mature. The fungus likes cool soil temperatures, and also goes to alfalfa, blackeye beans, wheat, cotton, and tomatoes.

In the San Joaquin Valley, Davis explained, cavity spot is usually caused by Pythium sulcatum and P. violae, although two other related pathogens, P. ultimum and P. irregulare, can also contribute to the disease.

At the moment, there is no way to quantify populations — which would be a way to predict disease risk — of the first two species in the soil.

“If we could take soil samples and confidently say that the population of P. sulcatum and P. violae is low and cavity spot incidence is going to be low, that could be significant in how you manage this disease,” said Davis.

In addition to reducing fungicide costs, this practice could limit the development of resistance to Ridomil and avoid the accumulation of Ridomil-degrading microorganisms in the soil.

This season, Davis is working with 20 carrot fields in Kern County to iron out procedures to detect the disease from soil samples.

Davis’ project began with attempts to use a real-time DNA test with small samples of soil to learn the amount the two major pythium species, but distinguishing those from the other two continues to be difficult.

He said the test failed 25 percent to 30 percent of the time because of toxins and other compounds in soil samples, so it was discontinued.

Instead, he is now taking larger soil samples, which are more accurate indicators, and milling them for uniformity.

Davis then uses the traditional method of placing samples in large petri plates laced with antibiotics to kill off other soil microorganisms and extracts pythium for DNA identification to measure populations.

Meanwhile, Jim Farrar, associate professor of plant pathology at California State University, Fresno, is evaluating fungicides at his campus carrot nursery in another project supported by the board.

Farrar said late spring rains delayed harvest of the experiments with fungicides and phosphorous/phosphoric acid-based materials.

“The delay may have allowed late-season cavity spot infections and erased differences in treatments,” he said.

But even with the delay in harvest, Quadris fungicide provided “an excellent level of cavity spot control.”

“Quadris is an azoxystrobin fungicide and is currently labeled for rhizoctonia root rot on carrot, but not for cavity spot. Phosphorous/phosphoric acid-based materials do not appear promising for cavity spot,” he said, although he added that the experiment might be worth repeating with hopefully normal harvest timing.

In a variety trial at the nursery, Farrar found a range of susceptibility to cavity spot. Among the ten varieties tested, Chactaw, Navajo, Legend, Apache, and Sun 255 had relatively higher disease incidence, while SixPak II, Chantenay Red Core, and Imperator had relatively lower disease incidence.

“Unfortunately, the least susceptible varieties are no longer commonly planted commercial varieties,” he said.

Farrar also reported on fungicide studies of alternaria leaf blight of carrots. He observed performance of Reason, one of the Group 11 fungicides which also include Quadris, Cabrio, and Flint, on the disease.

“These are very specific in their mode of action, and they are very susceptible to fungi developing resistance to them, so they have to be managed very carefully with rotation with other chemicals.”

Thus, he explained, when Group 11 fungicides are used for cavity spot, there is potential for developing resistance in an alternaria population.

He found Alternaria dauci, responsible for the blight, to be somewhat sensitive to high rates of Reason, but not to rates typically used (well below 10 ppm) in the field.

So, it doesn’t appear there are problems from using Group 11 fungicides for cavity spot and also using them for alternaria leaf blight, said Farrar.

The board also heard from David Tricoli, supervisor of the Ralph M. Parsons Foundation Plant Transformation Facility at UC, Davis, on recent transgenic work on root-knot nematode resistance in carrots.

The Mi 1.2 gene of tomato has long been known for its resistance to the soilborne pest and was incorporated from wild tomato plants into commercial varieties.

Tricoli’s operation plans to find 25 lines of carrots and once they are screened as suitable, the resistant gene will be inserted into them. The process uses a strain of Agrobacterium tumefaciens to carry the gene.

The project thus far has found five Imperator lines amenable to the gene transformation, Tricoli said.

Dave Woodruff, research coordinator for the board, noted the board has no plans to incorporate gene-splicing research into its program and the presentation was for informational purposes only.

Paul Gosselin, chief deputy director of the California Department of Pesticide Regulation, told the symposium this year will be “very significant” in new regulations for agricultural fumigants. New guidelines are expected to be in place in 2008.

CDPR has announced plans to place additional controls on pre-plant fumigants metam-sodium and metam-potassium, which break down into a volatile gas that may cause eye and respiratory irritation.

The reduction of volatile organic compounds (VOC), contributors to smog, is the key part of the department’s four-point initiative to improve air quality.

Additional areas of focus are reformulating other pesticide products to reduce emissions and risks, promoting new, more environmentally friendly technologies, and developing strategic pest management partnerships in concert with industry.

Gosselin said the VOC issue is changing daily as the department pursues detailed talks with industry in development of new proposed restrictions for metam-sodium.

In crafting the new regulations, he said, they want to balance computer models and science-based research with the “real-world” experience that growers have accumulated in using fumigants over the years.

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