Yesterday, I had a nice chat with Dr. Jim Adaskaveg, UC Specialist in Plant Pathology. As many of you may know, Dr. Adaskaveg conducts many of our foliar and bloom disease trials. Here were some additional comments to be added about bloom spray timing and last week’s post.
Dr. Adaskaveg highlighted that all parts of the flower are susceptible, but for the most part the flower petal is the most susceptible and is the primary point of flower infection.
Many of the fungicides used have what Dr. Adaskaveg termed “Reach Back Potential.” This basically means that a fungicide application controls the growth of infections that may have occurred BEFORE the fungicide application. In these cases, the fungicide does not serve as a curative, but rather as a fungal-static agent, or, in other words, it slows the growth of the fungus but does not kill it. This provides more time for fertilization and petal abscission to occur, thus reducing brown rot incidence.
He also pointed out that Brown Rot is most active in temperatures above 57 degrees F. It is important to note that infection can still occur below 57F, but the rate of growth is reduced.
He suggested that growers with brown rot tolerant varieties (i.e. Nonpareil) and no history of brown rot may be able to get by with a minimal spray program. In some cases he observed 50% Nonpareil orchards in which the grower did not spray the Nonpareil; fungicides were applied only to the pollinators. This practice was not recommended for Butte plantings. Butte is very susceptible to brown rot.
In regards to the upcoming weather and the beginning of bloom, he suggested holding the first spray until after the rains, trying to hit the 30% bloom mark on Nonpareil.This should provide coverage for the emerged Sonora blossoms. He felt comfortable with the ability to rely on the reach back potential of these fungicides to help reduce brown rot within the Sonora.
Keep in mind that Brown Rot is not the only disease of concern at this time. Green Fruit/Jacket rot occurs commonly on varieties that cluster tightly – Avalon, Wood Colony, Price, and Butte. Dr. Adaskaveg highlighted that Botrytis, one of the causal agents of Jacket Rot, will not be controlled by applications of FRAC 3 Fungicides (i.e. propiconazole, Quash). If propiconazole based products are planned to be used, he suggested tank mixing them with chlorothalonil in an effort to ensure sufficient disease control for Brown Rot, Jacket Rot, and Shot-hole.