A new working hypothesis on how bacterial blight develops in carrot-seed fields may help in finding improved management methods, say plant pathologists at Oregon State University.
Rhonda Simmons, Central Oregon Agricultural Research Station at Madras, described the findings during the annual meeting at Bakersfield of the California Fresh Carrot Research Advisory Board, which funds the studies.
The blight, caused by the several strains of Xanthomonas campestris, varies in intensity from one season to the next and can be controlled by hot water treatment of seed.
Although the treatment (soaking in water at 126 degrees F. for 25 minutes) neutralizes the bacteria, it also claims about 2 percent of the germination potential of the finished seed sold to carrot growers in California and across the U.S.
Research by OSU Plant Pathologists Fred Crowe and Simmons in recent years has led to some carrot-seed growers in the Pacific Northwest converting from sprinklers or furrows to drip irrigation to reduce losses from the blight, which is transmitted by splashing water.
However, their continuing studies focus on use of bactericides to protect fields, especially in seasons when rainfall is plentiful.
After trials during 2005-06, Crowe and Simmons reported that while antibacterial materials did not eradicate the disease, they did delay outbreaks until late in the season.
That, Simmons said, brought them to “a revised working hypothesis of how Xanthomonas develops in carrot seed fields.”
The new hypothesis begins with disease distributed in limited amounts in the fall by either wind, equipment, animals, seed, or other means. The pathogen develops on surfaces of some leaves and may spread during the fall.
Then during late fall or winter, a small amount, perhaps 5 percent, of the plants develop internal infections. Most of the surface infections die off during the winter, and fall antibacterial sprays may help prevent further internal infections.
“By the time carrot plants resume re-growth in the spring, all or nearly all Xanthomonas bacteria in the field are harbored in a small proportion of internally-infested plants,” Simmons says.
These bacteria cannot immediately move from plant to plant unless they are exposed by “undetermined events,” such as wounding or insect feeding.
“As the spring and summer develop,” Simmons says, “these undetermined events occur and some internally-located Xanthomonas does begin to spread among plants by typical means such as rain, sprinkler irrigation, plant to plant contact, or animals.”
Eventually, in unsprayed fields, very high populations develop.
A difficulty in dealing with the disease is that the crop can tolerate a significant amount of the bacteria without showing symptoms, only to flare up to very high levels when conditions are optimum.
Few of the various species and strains of the bacterial group are tolerant to extreme cold, which usually also kills exposed carrot foliage.
“In Central Oregon,” Simmons says, “we associate the worst Xanthomonas with earlier spring events such as hail and sudden, late-season frosts. Perhaps these are opening wounds that allow early and rapid epidemic spread.”
Their treatments with a combination of Mankocide with Tanos showed promise for reducing overwintering infections that might have otherwise expanded with high rainfall or humidity later.
Becky Westerdahl, Extension nematologist at the University of California, Davis, brought the board up to date on her recent studies in management of root-knot nematodes, which with stubby root and needle species, are an important group of pests of carrot.
In her search during for alternatives in the wake of the loss of standard chemical nematicides, Westerdahl listed materials that showed promise in 2006 and are in further studies this year. They were evaluated with Telone II because it has historically demonstrated the most consistent nematode control.
Two trials were at the UC Shafter Research and Extension Center at Shafter and three were at the UC South Coast Research and Extension Center at Irvine.
The Valent microbial product, DiTera, is thought to work against nematodes by stopping feeding and egg production. Westerdahl said she has been working with the material for several years, with “variable but generally positive results.”
Abamectin seed treatments are being developed by Syngenta for several crops, including carrots. The compound, currently marketed as a cotton seed treatment under the name Avicta, is produced from the soil micro-organism Streptomyces avermitils. It also has application in combating nematodes in veterinary medicine.
A contact nematicide, it interferes with nerve cells of the nematode and prevents feeding and reproduction. Westerdahl said it has been tested as a soil treatment, but as a seed coating it can move through the soil profile more readily.
The material is thought to be most effective in the first few weeks after planting before it degrades in the soil.
The product Quillaja 35% is an extract of the soap bark tree and is a widely used food additive to produce foam in soft drinks and also in shampoos.
As one of the emulsifying saponins, it is thought to move through the cuticle of the nematode, making for easier penetration by toxins. It may also prevent production of compounds required by nematodes for growth and development.
“In this year's trial at Shafter,” Westerdahl says, “the DiTera + Quillaja treatment has a greater number of marketable carrots than the untreated.”
Stoller USA has embarked on an approach of manipulating plant hormones to suppress nematode feeding on roots. Root Feed is intended to make roots more vigorous, increase disease resistance, enhance flowering, and promote even crop development.
Sugar Mover Plus, Westerdahl says, is designed to redirect the flow of sugars from the leaves to the fruiting parts of the plant and to promote more efficient conversion of nitrate nitrogen for possible reduction of excessive vegetative growth.
The plots at Shafter showed a treatment with a combination of Root Feed and Sugar Mover to have a weight of marketable carrots comparable to Telone II.
According to Westerdahl, even when nematode-resistant carrots are achieved, other control methods will still be needed. Continuous planting of resistant varieties has historically led to resistance-breaking races, so rotations with susceptible varieties will be advisable.
Furthermore, she noted, resistant varieties are likely to be developed only for root-knot nematode, whereas current chemical methods control all three major nematode pests of carrots.