Evaluate Your 2022 Landscape Disease Management Program Now

Landscape managers should take the opportunity to walk the landscapes under their care this fall to look for signs of disease on the foliage of plant specimens. Signs of fungal and bacterial leaf spots are often very visible at this time, and they may provide the landscape manager with some insight into how effective their spring/summer protective fungicide/bactericide applications were.

Most foliar diseases will not kill healthy, vigorous plants directly, but years of repeated infection coupled with premature defoliation can leave some plants vulnerable to attack by other organisms. Protectant fungicides/bactericides applied in the spring/summer to prevent infection may be "washed off" by heavy rains leaving these plants unprotected and susceptible to infection.

Fungicides used to manage foliar diseases are deemed more effective when applied prior to rain. But as a rule of thumb, one inch of rainfall will remove 50% of the protectant fungicide residue from the foliage, and two inches of rainfall will remove close to 100% of the fungicide residue. When a 2-inch rainfall event occurs post-application, the treated plant materials will essentially have no protection from infection.

Landscape managers often cannot drop everything and re-treat landscapes after every heavy spring rain event, but with a little foresight and planning, managers can select protectant chemistries that have good rainfastness. Rainfastness is defined as a pesticide's ability to persist after a rain event. Rainfastness is not equal for all pesticide products and/or formulations. Landscape managers should carefully weigh their treatment options when considering the plant species, the disease to be managed, and the customer they serve.

Some customers understand that rainfall can reduce or eliminate the ability of a fungicide to provide protection, while other customers may threaten litigation if one blighted leaf on a plant is observed. Two years ago, a litigious property owner threatened one landscape company in my area because Douglas fir trees were defoliated by Rhabdocline needle cast. The applicator, in this case, selected the appropriate chemistry and applied the chemistry at the labeled rate three times at prescribed intervals. A heavy spring rain that caused minor flooding in the area struck two days after the trees were treated the first time, which left the trees largely unprotected and vulnerable to infection. Subsequent applications of protectant fungicides were made to this line of Douglas fir trees by the applicator, but unfortunately, the trees had already become infected after the first rain event, and defoliation was observed. Perhaps the selection of a fungicide formulation with better "rainfastness" may have yielded better results for this applicator.

Selecting and utilizing some pesticide adjuvants or "stickers" with some plant protectant chemistries may also help extend a pesticide's efficacy when frequent rains are forecast. The use of pesticide adjuvants or stickers with protectant fungicides should always be governed by the pesticide label. In some cases, the addition of a pesticide adjuvant to the spray tank can increase the risk of phytotoxicity with some plant species.

Protectant fungicides are best applied during sunny, dry conditions to allow for quick drying on the foliage. When protectant fungicides are applied under dry, sunny conditions, the fungicides are better absorbed by the leaf surface and are considered more rainfast.

Systemic fungicides are best applied under humid, cloudy conditions when the soil is moist and when the leaf cuticle is more receptive to the absorption of the systemic fungicide. Under hot, dry conditions, the leaf's cuticle is considered less permeable. Any fungicide residue that remains on the leaf surface will be subject to both UV and microbial degradation and may be more subject to removal by rainfall (Rainfastness of Fungicides).

As autumn slowly marches in, take the time to walk the landscapes under your care and note the presence of any serious foliar disease problems that you observe. Have frank discussions with the property owners about your observations and how you will manage their landscape in the coming year. When you develop your disease management program for 2023, select fungicide products that offer both good rainfastness and heightened efficacy against the diseases that you are trying to control.