written by Dr. Jim Adaskaveg (UC Riverside), Brent Holtz (UCCE San Joaquin), Roger Duncan (UCCE Stanislaus), and David Doll (UCCE Merced).
In the spring of 2013, we reported a high incidence of bacterial spot in some Sacramento and northern San Joaquin Valley almond orchards, especially on the ‘Fritz’ variety. It was also found on ‘Nonpareil’, ‘Butte’, ‘Carmel’, ‘Monterey,’ and ‘Price’ but at much lower levels. The disease has not been found in orchards in the southern San Joaquin valley.
Bacterial spot is caused by Xanthomonas arboricola pv. pruni and has also been referred to as bacteriosis, bacterial leaf spot, or bacterial shot hole. Bacterial spot occurs on leaves, twigs, and fruit of almost all Prunus spp. The pathogen commonly causes bacterial spot of peach in the eastern United States and is one of the major foliar diseases in high rainfall years. On almonds, it is a significant problem in Australia because of mid-season rain events. In 2014, we verified the presence of the disease at previous and additional locations in California, although at levels much lower than in 2013.
Symptoms on almond may develop on leaves and shoots, but the most obvious symptoms are on fruit. Typically, almond hull lesions start as small, watery blemishes that produce light to dark amber gumming. Lesions are brown and slowly increase in size to 2/32 to 5/32 inches (2 to 4 mm, generally <5 mm) in diameter during the season as the infection extends into the hull. The amber color of the gumming is important because this helps distinguish bacterial spot from the clear gum of leaf footed bug feeding injury. Infections starting early in the season can cause fruit drop and infections that reach the kernel may cause off grades, or fruit may be un-marketable. Symptoms are usually first visible 7 to 21 days after infection with rapid expression dependent on warm temperatures. Small angular lesions may develop on leaves at the leaf tip, mid-rib, or along the leaf margin. Infected leaves may prematurely drop. As lesions age, centers may darken, become necrotic, and they may abscise. Leaves are most susceptible before becoming fully expanded. Twig lesions may develop on green shoots. These lesions are not obvious on almond but have been reported on almonds in Australia. On peach in the eastern US, they often turn into elliptical cankers.
Epidemiology. The pathogen is readily isolated from overwintering symptomatic fruit mummies well into the spring season, indicating their role as primary inoculum sources during infection periods. Twig lesions and bud death were not observed in our studies but have been reported on other Prunus spp. In the spring of 2014, wet and warm conditions did not occur until mid- to late March, and fruit symptoms were first observed in mid- April. Infections appear to initiate on developing fruit during or immediately after shuck split under favorable conditions. The bacterial pathogen needs splashing water to spread and wet conditions to infect. On other crops in the eastern United States, high winds during rain generally cause more severe infection periods.
Management. Strains of the pathogen collected and evaluated to date were all rated as copper-sensitive with growth occurring at 20 ppm copper, but not at 30 ppm. Field trials on the management of the disease were conducted that included dormant and in-season applications. Late-dormant treatments (late January) with copper, copper-mancozeb, or other combinations significantly reduced the incidence of disease, but not early dormant treatments that were applied in mid-November or mid-December 2013. In-season treatments were most effective when timed around rain events and before temperatures started to increase above 77ºF (25°C) in the springtime. Copper treatments were applied by halving the rate with each successive application (e.g., 1, 0.5, 0.25 lb MCE/A – metallic copper equivalent / acre). No copper phytotoxicity was observed after four applications when copper rates were successively reduced.
All copper products significantly reduced disease. Among products evaluated, however, Kocide 3000, Badge X2, Champ-Ion2+, and Cuprofix were more effective than Cueva. A single delayed dormant (copper-mancozeb) and a single in-season treatment (copper alone or copper+mancozeb) in mid-March timed before rainfall and warm conditions provided excellent control and was equivalent to five applications (delayed dormant and four applications in 10- to 14-day intervals starting at full bloom) using the same chemicals.
Other experimental treatments with high efficacy included Kasumin-Manzate, Kasumin-Captan, Kocide-Tanos, Mycoshield/Fireline (oxytetracycline; federally registered on peach for this disease), Serenade Optiva, and the new bacterial membrane disruptor Ceragenin. Kasugamycin was accepted into the IR-4 program in 2014 for registration on almonds and peaches in the United States.
Summary. Bacterial spot is a new bacterial disease of almond in California that is caused by Xanthomonas arboricola pv. pruni. The disease mainly occurs on cultivar ‘Fritz’ in the mid- to northern almond production areas of California and commonly develops on fruit. Based on our first year of research, the pathogen primarily overwinters on diseased mummified fruit and infection periods are during warm, wet conditions during the spring as fruit and leaves develop. Our results from this past season indicate that the most effective management program for bacterial spot includes a delayed dormant (Jan.) bactericide (e.g., copper-mancozeb) application to reduce inoculum and at least one in-season application around rainfall events and rising temperatures (e.g., mid- to late March) to prevent new infections. In a wet spring, additional in-season applications may be needed to protect developing fruit. Most fixed-copper products and copper-mancozeb treatments are highly effective. To validate this, field trials will be conducted again in the coming season.