Preventive Control

Bal Rao, Ph.D., manager of research and technical development for The Davey Tree Expert Co., headquartered in Kent, Ohio, has treated his share of ornamental diseases over the years. From rust and scab to leaf spot, Dr. Rao is among the industry leaders in research for methods to control these destructive fungi. Through the years, one of the key components to Rao’s research has been determining the best timing for fungicide applications.

PREVENTIVE VS. CURATIVE. Rao has seen best results with ornamental disease control when products are used on a preventive basis.  “Control of ornamental diseases is difficult when using a curative application,” says Rao. “To ensure the health and continued growth of a tree, a preventive application offers the best chance of success.” 

Recently, Rao and his team studied the benefits of a preventive fungicide application on plots of crab apple and hawthorn trees with scab and rust problems. Rao and his team used a control group of untreated crab apple and hawthorns and a group treated with preventive fungicide applications for the trial.

The untreated crab apple test plot showed visible infestation of scab – a fungal infection identified by discolored lesions on the leaves and fruit. Similarly, the plot of untreated hawthorn trees suffered significant rust growth – a fungal infection identified by discoloration of the leaves, typically from yellow to orange. In both untreated groups, more than 70 percent showed signs of infection.
“While some fungicides can be used for curative control, once a disease or pest has infested a tree, there is not much time for action,” stresses Rao. “If no fungicide has been used, disease will lead to defoliation.”

“In both cases, the fungus is clearly visible through discoloration of the leaves and fruit, leaving it unsightly and resulting in a reduction in the leaf-count of the tree, making it vulnerable to winter injury or other diseases.”

To protect itself and maintain photosynthesis, the tree must produce new leaves before the season is complete. But, to do this, the tree must exert additional energy, leaving it susceptible to additional diseases in the process. This is where the value of preventive application is evident.

The test groups of crab apple and hawthorn trees were treated with a preventive application of Armada™ fungicide. Applied by foliar spray, the fungicide was used at rates of 3, 6 and 9 ounces per 100 gallons. The product was applied about every 28 days for the crab apple trees on April 17, May 17 and June 7; and twice for the hawthorn trees on May 25 and June 26. 

As rates increased, researchers noted a correlation to the disease controlled. “Results at the rate of 3 ounces yielded fair control, much better than that of the untreated ornamentals. However, the success rate increased dramatically at the higher application rates,” says Rao. (At higher rates, some trees displayed no symptoms at all.)

28 DAYS OF CONTROL. A typical fungicide has between a 10- and 21-day residual control, leaving trees with a gap in coverage. If the tree produces new growth following residual control, it receives no additional fungicide protection, making it susceptible to new diseases. This is where the benefit of a product with a longer residual, such as Armada fungicide, becomes apparent. “The 28-day residual achieved with Armada is ideal for protection in the early growing season, when ornamental diseases like scab and rust are at their most destructive stage,” notes Rao.
 
With protection against disease early in the season, the tree is less susceptible later when diseases can be just as damaging if the tree is already injured.