Scab Spray Timing and Other Spring Disease Questions

Wet weather has found its way into the valley, with rain events predicted off-and-on for the next week to 10 days. I have had a few questions regarding fungicide applications.Chlorothalonil (i.e. Bravo) isn't registered for jacket rot of almond? Should I tank mix in another fungicide to provide protection?

Figure 1: Overwintering scab lesions on thegreen wood of almonds.

That is correct - Almond jacket rot is not on the label for chlorothalonil. Therefore, if a recommendation is written for jacket rot, another labeled fungicide must be used. FYI - chlorothalonil is labeled for jacket rot of apricot, which is caused by the same pathogens (Botrytis cinerea, Sclerotinia sclerotiorum, Monilinia laxa,Monilinia fructicola).What is the best time to apply a fungicide for Scab?Scab (Cladosporium carphophilum) overwinters on twigs of almonds (Figure 1) and will re-infect new leaves shortly after leaf out. Control of this diseases has been documented with dormant applications, petal fall applications, and applications made 2-5 weeks after petal fall. Earlier post-petal fall applications (~2 weeks) provides more effective control than later applications.

Figure 2: Scab lesions sporulating on an almondbranch. Note the darkened circles within thelesion. Fungicides should be applied for protectionwhen this is observed to prevent scab infection.Photo courtesy of the Almond Board of California.

The timing of the fungicide application should be made when the overwintering scab lesions begins to sporulate (release spores into the orchard).  When sporulating, the lesions, which are found on the green twigs of the tree, will develop a dark ring (Figure 2). Providing protection of the leaves with fungicides at this point will help reduce/prevent scab infection of the almond trees.Is there any way of determining if we are going to have another bad year of rust or scab?Predicting disease epidemics is challenging. Basic epidemiology indicates that in order for disease to occur, we need to have a susceptible host, a pathogen present, and environmental conditions that favor disease. Over the past two years, there have been many problems associated with scab and rust, indicating that the pathogen and susceptible hosts are present within the orchard. The third variable, the environmental conditions, is always changing, with some years favoring disease. The springs of 2010 and 2011 have been wet and cool, followed by mild summers.  This is in contrast to low rainfall springs and hot summers of 2007, 2008, and 2009. The wet, cooler, and more humid than normal conditions experienced over the past two years has, most likely, been the primary driver behind the many foliar disease problems experienced.Keep in mind that various strategies can be implemented to reduce disease by manipulating the micro-climate of the orchard. This includes wider tree and aisle spacings and proper irrigation timing and application, which will reduce orchard humidity, as well as planting more resistant varieties to the pathogens. All strategies have trade offs, which may lead to lower potential yields.