The ag chemical industry, university researchers, PCAs, and growers all have key roles in vigilant management of resistance of fungal diseases in California tree crops, says a professor of plant pathology at the University of California, Riverside.

James E. Adaskaveg, who specializes in foliar diseases of almonds, citrus and stone fruit, addressed new fungicidal formulations and how to maintain their efficacy during a recent conference held by Bayer CropScience in Monterey.

Although new fungicidal products have promise, he said, “We have to keep our guard up for pitfalls in introduction of new materials and methods, as strategies are developed for future use of them.”

Noting that fungal organisms are quite dynamic and can develop resistance to fungicides rapidly, he called for an effort by all parties concerned.

In describing what each group can contribute, Adaskaveg said ag chemical companies are developing new and improved chemistries, while universities are working out disease control strategies to best use the products for the long term.

“And the role of PCAs and growers,” Adaskaveg added, “is to promote fungicide stewardship, even if it means using a more expensive program, by rotating materials and using labeled rates to prevent resistance.”

The objective, he stressed, is to “minimize pathogen survival” by avoiding repeated exposures of the same class of fungicides to pathogens, especially in situations of high disease populations.

Among new fungicides recently available or expected to be available soon to combat anthracnose, scab, shothole, brown rot blossom blight and other pathogens, are “pre-mix” formulations, including the Bayer products, Distinguish and Adament, that combine two active ingredients having different modes of action against the target pathogen.

“Overall, the pre-mixtures are a good thing for the industry, but we still have to be smart and use them to get the most from them,” he said.

During the past few years, Adaskaveg has been conducting some 30 trials annually to evaluate fungicides at various locations around California — particularly sites where the disease problems are the worst.

He said his trials emphasize observing the biology of the foliar and blossom fungal organisms to understand where they occur and persist in the trees as a means of learning how to manage them. In addition to fungicide applications, this broad approach includes developing orchard sanitation practices to apply additional pressure against a pathogen by breaking up its life cycle.

For trials in almonds Adaskaveg has typically selected orchards having a worst-case scenario of the most susceptible varieties with the highest levels of fungal disease to challenge the relative performance of fungicides.

In the case of brown rot, researchers have learned the importance of fruiting bodies of the pathogen residing during winter on the orchard floor and lower trunks until spring weather activates them.

While fungicides with a single mode of action can offer control, he has found greater protection from the pre-mixture products. “We have seen improved disease control across-the-board, but we feel that labeled rates of the pre-mixture products are critical for maintaining their performance.”

Adaskaveg went on to say these products are a natural evolution of new classes of fungicides that, because of their multiple modes of action, reduce the potential for diseases to develop resistance.

He and his research colleagues have been evaluating the products and determining the best timing and rates. They plan to issue their results to growers on University of California Web sites.

The combination products are all the more important because they are appearing in the wake of recent discoveries of resistance to single-chemistry classes of fungicides.

For example, Adaskaveg said, resistance to strobilurins and carboximides turned up in Alternaria leaf spot in 2003. In many orchards resistance to strobilurins was found in almond scab in 2006.

“And the big, bad news is that we found brown rot resistance to anilinopyrimidines in one orchard. Resistance to fungicides is real, and it is not hype.

“We have to be careful to develop and maintain strategies to keep the new materials. Once you loose a class of materials, all the products in that group will go out,” he said.

Another speaker at the gathering, Larry Bettiga, Monterey County farm advisor, said the vineyard pest, powdery mildew, calls for a knowledge of how the disease works, backed up by a planned rotation of fungicides, for growers to stay a step ahead of it.

“Powdery mildew is probably our number one disease on grapes, and it's essential growers recognize how the disease starts in the springtime to maximize their spray program against it,” he said.

Most Central Coast infections start from the disease's cleistothesia, washed onto trunks through spring rains. It can then lurk for months until ignited by warmer temperatures.

Control, he noted, is much easier when vine canopies are small and more easily thoroughly covered in a treatment. “When leaves have a shingling effect on sprays, it is almost impossible to get material to the clusters.”

The two critical periods are post-bud-break and again from pre-bloom to bunch closure. Although varieties vary in susceptibility, in certain years with weather favorable to the disease, it can strike in any vineyard.

“Mild temperatures in the 70s to mid-80s are the main drivers of powdery mildew, and hopefully you can spot it and prevent it before it becomes a major issue,” he said.

When infection is slight, many materials may perform well by themselves, but when the amount of infection becomes heavy, care has to be taken to develop a broader program.

Although an array of fungicides is available, he said, each class works in a specific way and rotating different classes of products is the answer to managing the shifts in the disease's susceptibility to them over time.

“You need to limit the number of applications by season of any single fungicide class and mode of action, and you want to maintain the labeled rates, even in a preventive program.

“If you use a low rate early, you may have enough mildew escapes so that in an epidemic situation you end up with a fair amount of infection by harvest time,” Bettiga said.