Nurseries are using drip irrigation and other practices to reduce Anthracnose on strawberries, and growers in California’s coastal counties can take additional steps to better manage the disease, according to Mark Bolda, Santa Cruz County farm advisor.
At recent plant disease seminar in Salinas, Calif., he described a trial in Camarillo comparing drip irrigation with sprinklers on strawberry plants intentionally inoculated with the soilborne disease.
The study revealed that the free water from sprinkler irrigation encouraged the fungus, causing plants to be significantly smaller and marketable yields to be less. Infections occur when soil containing the disease’s spores is splashed onto the crowns and stems of plants by rain or irrigation water.
“If the mother plants have Anthracnose, the irrigation method can make a difference, especially in plant survival,” Bolda said.
Observations from his similar study done in Watsonville are being tallied, and he said he anticipates a similar outcome from them.
Since infections can be in nursery soils, nurseries in the Macdoel area of Northern California are taking pains to remove soil contaminated with the disease from strawberry transplants as they are harvested.
Growers can continue pressure against Anthracnose by dipping transplants in solutions of Abound or Quadris fungicide for two to five minutes immediately before setting them in the soil.
“Don’t dip and wait, plant them immediately, and remember that you have to change the solution frequently to remove the accumulated soil in it. That can be expensive, but the fungicide sticks to the soil, and once that happens it is not going on the transplants,” he said.
An alternative for organic production is simply dipping transplants in clear water before putting them in the soil. It will remove soil from roots and reduce potential for the disease.
When fungicide treatments for Anthracnose are called for on the foliage, several products are available, and, Bolda stressed, it is important that the field also be strong with proper plant spacing, plant chill, fertility and irrigation.
Anthracnose can be spotted by wilted and collapsed plants, but dark-colored lesions on stems and runners occur beforehand. In warm, humid conditions, salmon-colored masses of spores form on lesions.
The pathogen also causes fruit decay, especially after warm, rainy weather, and fruit at any stage of ripeness can be affected by small, oval-to-round, brown spots that remain firm and dry.
Its spores can survive in soil for at least nine months without a host. In addition to strawberries, several weeds such as chickweed, fiddleneck and vetch are hosts.
Recalling the outbreak in 2003 of multiple viruses in strawberries – particularly short-day varieties – in the Watsonville area, Bolda said growers can detect infected areas by reddened foliage.
A complex of viruses, including pallidosis and beet pseudo-yellows, is vectored by whiteflies and aphids, and Bolda said one of the best practices is to apply Admire via drip tape after transplanting.
The systemic insecticide has a 60-day preharvest interval, and must be applied “sooner rather than later.” Care should be taken during irrigation to give the material enough time to move into the root zone for uptake by the plants.
He said he has seen strawberry leaf blotch in coastal fields during the past three seasons. It shows first as gray lesions on leaflets of transplants. Infections expand from the edge of leaflets and are surrounded by irregular purplish-red borders.
The disease survives in strawberry residue in the soil, but in the past three seasons it has disappeared with the advent of warmer temperatures. “Don’t worry about it. Once the rain stops and the weather warms up, it’s gone,” he said.
Shifting to diseases of caneberries, Bolda said orange rust on blackberries is the most significant and can be identified by its orange pustules on the undersides of leaves. New leaves of infected plants are stunted, deformed and yellowish.
“The important thing to remember about this disease is it is systemic. It is in the canes, the crowns and the roots, and no fungicide will control it.”
The only solution is to rogue out the infected plant and destroy it before the disease spreads to other plants.
On raspberries, yellow rust is known to growers, but Bolda said a similar but separate rust species has recently been found and is being investigated.
Rally and Pristine fungicides have performed best in his continuing raspberry trials. He noted that blackberry rusts and raspberry rusts do not cross-infect.
Tom Gordon, plant pathologist at UC, Davis, told the seminar attendees genetic resistance in strawberries cannot equal preplant soil fumigation for control of Verticillium wilt.
Verticillium dahliae, the pathogen responsible, is indigenous to and widespread throughout California and attacks many crops. If not destroyed, its microsclerotia can remain viable in the soil for years.
As the strawberry industry looks to the future and prepares for anticipated loss of fumigants as controls, he said, other methods – including genetic resistance – will become the focus in managing the fungal disease.
With support from the California Strawberry Commission, Gordon and others at UC, Davis set up vert trials at the Watsonville Experimental Station.
Transmission of the disease from mother plants to daughter plants was traced in the trial. One problem is that while mother plants show outward signs of infection, the disease can be present in daughter plants without symptoms being readily visible.
The researchers learned that rates of infection by V. dahliae isolates from potato, cauliflower, and other host crops vary among strawberries genotypes.
“But,” he said, “we really don’t have something so resistant that we can suggest it is a substitute for fumigation.”
Even so, he noted, the strawberry breeding program uses susceptibility to vert as a criteria in the selection process for new germplasm.
Breeders discard highly susceptible materials, and the selection process each year is moving toward resistance. For example, the new release, Albion, has much more resistance to the disease than Camarosa.
Soil fumigation, Gordon said, has a special effect that is not well-understood. “We’ve never entirely figured out what it is caused by. It may be because of a change in the microflora where we get rid of a lot of minor pathogens or maybe it is a subtle fertility effect. No one has ever managed to reproduce it by any other means.”