Alternaria Late Blight of Pistachios
Phoebe Gordon, Orchard Crops Farm Advisor, Merced County
Alternaria Late Blight (ALB) of pistachios is a fungal infection caused by several Alternaria species, Alternaria alternate being the most common. These species can also cause Alternaria leaf spot in almonds, which tends to show up in the summer. Severe infections will result in leaf loss, and the fungus will move into the fruit, causing staining and molding and thus lower nut quality. This is unlike almonds, where Alternaria infections only damage the leaves, though defoliation can be severe if the disease is unchecked.
Alternaria infections have differing symptoms depending on whether infections happen on the leaves or fruit. Leaf infections can be angular or circular and start out small with either dark brown or black coloration. As they grow, they will merge together and form light brown spots that can be as large as an inch. When humidity levels increase the fungus produces spores on the surface of the lesions which results in them blackening. You can differentiate ALB from Botryosphaeria panicle and shoot blight by rubbing your fingers on leaf or fruit spots: the spores from ALB will coat your fingers. Alternaria infections on pistachio fruit and panicles remain small – no more than a few millimeters in diameter, and these spots stay black. Red haloes will surround the lesions on immature fruit, some of which will grow and merge.
Like most foliar fungal diseases in orchard crops, ALB thrives in high humidity conditions generated in dense canopies, flood or sprinkler irrigation, or from nearby bodies of water. Sodium affected soils, which reduce the rate of water infiltration and can result in water being ponded on the soil surface for longer periods of time, can also increase humidity levels in the orchard. To combat a severe ALB problem, you must address orchard humidity levels in addition to your spray program. Alternaria evolves resistance to fungicides readily, so reducing sprays ensures that existing fungicides can remain effective.
Lowering orchard humidity can be achieved by pruning to open the orchard canopy, which in turn increases air flow. More effective measures are to address irrigation delivery and management. While shifting irrigation systems can be a pain, ALB is a problem that can absolutely linger in orchards if humidity levels are not addressed. Severe ALB infestations can result in losses of up to $1000 per acre, so fixing a problem block can repay itself quickly.
If the orchard is irrigated with flood or sprinklers, conversion to microsprinklers or drip decreases the surface wetted area, which reduces orchard humidity. Ensure you are not overirrigating by monitoring evapotranspiration, and only applying what the trees lose. If possible, decrease the frequency of irrigation and irrigate more deeply, though do not irrigate for longer than 24 hours in an irrigation set to reduce the danger of Phytophthora infections. In past studies, subsurface drip irrigation managed in such a way that surface wetting was minimized reduced ALB in a severely affected orchard (Goldhamer et al., 2002).
Severely affected orchards will need sprays in addition to lowering orchard humidity; getting the problem under control cannot rely on one method alone. Early June is the time to start treatments, and three applications are recommended. There are a wide variety of available fungicides, so if you decide you need to conduct multiple sprays, ensure you rotate chemistries, and do not bring a mode of action back into the orchard in a growing season once it has already been sprayed. For instance, if you spray with a group 11 fungicide, conduct your second spray with a group 3 fungicide. Changing the formulation is not enough. For a complete list of fungicides and their FRAC groups, visit http://ipm.ucanr.edu. As always, check with your PCA or pesticide manufacturer so that you are up to date on which fungicides are registered for use in pistachios.
Cited open source article:
Goldhamer, D.A., T.J. Michailides, and D.P. Morgan. 2002. Buried drip irrigation reduces fungal disease in pistachio orchards. California Agriculture. 56(4): 133-138