Alternaria leaf spot and almond rust are fungal diseases of almond that are becoming more prevalent in the Sacramento Valley. Both diseases are favored by high humidity and leaf wetness. Often, additional fungicide treatments are necessary to minimize early defoliation. Recent extended wet springs and changes in cultural practices (higher density plantings and microsprinkler irrigation with longer, more frequent irrigations) are contributing to higher humidity and more accumulated leaf wetness hours (e.g., dew, rainfall, irrigation, etc.) resulting in higher disease incidence.
Alternaria leaf spot is a fungal disease caused by a complex of Alternaria species including A. alternata, A. arborescens, and A. tenuissima. Alternaria leaf spot appears as up to half-inch diameter brown spots on leaves. Leaf spots turn black as the fungus produces spores. Alternaria leaf spot develops most rapidly in the hot summer months, and can almost completely defoliate trees by mid-summer.
Relying entirely on fungicides to control this disease can be costly and increases the risk of resistance development. Consider an integrated approach including:
• Planting less susceptible cultivars. Varieties most susceptible include Carmel, Sonora, Monterey, Winters, and Butte.
• Select a planting design which allows for air circulation. Orchards planted with rows in an east/west direction typically have more severe disease than orchards with rows in a north/south orientation.
• Prune and train trees to allow air circulation and reduce dew formation.
• Practice good foliar disease and mite control to minimize stressed and injured leaf tissue.
• Irrigate less frequently with larger volumes of water to minimize relative humidity and subsequent leaf wetness.
• Manage the orchard floor to reduce relative humidity and the amount of senescing tissue colonized by Alternaria species.
Disease resistance against QoIs (strobilurins; Fungicide Resistance Action Committee — FRAC — group 11) and SDHIs (FRAC group 7) occurs in the Sacramento Valley. Late-spring/early-summer applications should alternate materials to manage resistance. New materials such as Quash (Valent USA), and Inspire Super (Syngenta Crop Protection), both containing FRAC group 3, and Ph-D (Arysta LifeScience, FRAC group 19) must be used in rotations and mixtures for resistance management. Newer SDHI fungicides (different subgroups) are proving to be highly effective, but the potential for resistance is also extremely high. Combination tank mixtures, pre-mixtures, and rotations will be required for preventing disease resistance to the newer SDHI compounds.
Rust is caused by the fungus Tranzschelia discolor and occurs sporadically throughout almond-growing areas in California. It appears as small, yellow, angular spots on the upper surface of leaves and rusty red pustules of spores on the lower surface. The disease is favored by spring and early summer rains and is more likely to become serious in orchards near rivers or streams or other locations where spring and summer humidity is relatively high. Excessive levels of nitrogen are also known to increase a tree’s susceptibility. The disease causes premature defoliation and will weaken trees, reducing the following year’s bloom. The rust fungus overwinters in infected leaves that remain on the tree, spores contaminating buds and tree bark, and possibly infected twigs. Rust is frequently more severe in young vigorous trees, especially in second to fourth leaf orchards where fungicides have not been applied.
In orchards with a history of rust, treatments should be applied before symptoms appear: five weeks after petal fall, and a second application four to five weeks later to control leaf infections. Two or three applications may be needed in orchards that have had severe rust problems. A zinc nutritional spray (zinc sulfate 20 to 40 pounds per acre) applied in late October to early November resulting in defoliation may reduce overwintering rust inoculum.
Watch For Resistance
Resistance management will be critical to maintain efficacy of currently available fungicides. Resistance development in Alternaria species to QoI fungicides was first detected in 2003/04. Field disease resistance was found in Kern County in 2005 and in northern California in 2007. Field disease resistance to SDHI fungicides (group 7) was found in the northern and southern Central Valley in 2007. For rust, resistance has not been detected and the potential for resistance against QoI (group 7 or QoI) and DMI (group 3) fungicides is considered low.
The following are general suggestions for fungicide resistance management:
• Rotate and mix fungicides that belong to different FRAC group numbers.
• Apply per-acre label rates, no every-other-row spraying (upper label rates for QoIs).
• Limit a single mode of action fungicide class application to one or two per orchard per season.
• Start your fungicide program with a multi-site mode of action material (Captan; Bravo/Echo, Syngenta; Ziram, Bayer CropScience; Rovral; sulfur). Sulfur can
be used in combination with single-site mode of action fungicides such as QoI and DMI fungicides.
Fungicides effective for Alternaria leaf spot and rust can be found at www.ipm.ucdavis.edu. Click on Agricultural Pests, then Almond, and then the individual diseases. Another resource is the “2012 Efficacy and Timing of Fungicides Publication” at http://ipm.ucdavis.edu/PDF/PMG/fungicideefficacytiming.pdf.