This past week, rain showers hit across the state, extending as far south as the Madera/Fresno area. In the Merced area, rain was variable, with most reports indicating about a 0.5″ of rainfall.I have had a few questions regarding the the need for another fungicide spray for the summer diseases of Rust and Alternaria. The following conditions and considerations should be kept in mind when deciding the need for another spray:
Orchard History. If defoliation occurred last year due to these pathogens, there is a good chance that the over-wintering inoculum levels are high, increasing the chance for an epidemic this year. This factor remains important even if sprays were applied earlier in the season.
Conditions post rain event. The hours and days after the rain were windy and warm, which dries the orchard. These conditions helped reduce the duration of leaf wetness, and canopy humidity, creating an environment that would be less favorable for disease.
Timing of last fungicide spray. If a fungicide was applied in late May due to a delay on the “May Spray,” there is a good chance that there is enough residual control of the material. Sprays of DMIs and strobilurins provide protection of covered tissue for 14-20 days in conditions without rain. If a fungicide hasn’t been applied since the late April/early May, a spray should be considered due to the lack of leaf protection. Any new growth that occurred since the last spray will not have adequate fungicide protection.
Tolerance of disease. My perspective in managing foliar diseases is to achieve the goal of having active leaves present on the tree in late October. In other words, there can be some disease present on the trees, but the tree has not defoliated and still has the ability to photosynthesize. Many people have a different threshold, which usually includes a lower disease tolerance. If you do not want to see any disease on the tree in the fall, a spray should be considered.
Material for use. Strobilurins and DMIs are very effective in controlling Rust. Microthiol can, and in my mind should be considered for rust control IF no oil has been applied with previous in-season sprays (i.e. May Spray). In regards to Alternaria, plan to rotate chemistries as frequently as possible. DMIs and strobilurin chemistries make up the back-bone of Alternaria control, with many products containing either a single or double mode of action. The double mode of action fungicides are very effect in control. Ph-D is a new, medium resistance risk chemistry that can be mixed into the rotation.