Management of Rusty Spot and Bacterial Leaf/Fruit Spot of Peach

1. Some locations are reporting occurrence of Rusty Spot of Peach, a usually rare disease in most orchards. This is a fungal disease developing on peach fruit only thus leading to losses and it is caused the same pathogen causing apple powdery mildew - Podosphaera leucotricha. The fungus either probably overwinters in peach buds, or trees get infected from nearby apple trees as the source of infection (conidia i.e. spores of the fungus are spread to peaches by wind, dew droplets, and rain). Critical timing for application of fungicides is from shuck split through 3rd cover (pit hardening) In other words young peach fruit are susceptible and get infected if protection is not timely applied. However, the infections can start as early as petal fall. If you are scouting orchard frequently, finding the first white or rusty patch spot of this disease should be the trigger for an immediate fungicide application. The most effective fungicides are Gem, Luna Sensation, Merivon, Fontelis, Tilt, Elite, Inspire Super, Rally, Indar but keep in mind that Rally will not give good control of scab on peaches. Sulfur has good efficacy but requires more frequent application than single site fungicides. Combinations of Topsin and sulfur or Topsin and Captan have fair to good efficacy. When using single-site fungicides do not apply more than four application per year of fungicides belonging to the same group and strive to alternate fungicides from other groups i.e. with other modes of action every week or a fortnight. This will reduce chances for development of fungicide resistance in P. leucotricha fungus to any of the groups individually, which if present would be a problem in both apple and peaches. Additionally, do not exceed the total number of sequential applications listed on the label (e.g. Group 7 fungicides aka SDHI-s) or their total number per season. Most cultivars are resistant to this disease but few examples like Rio-Oso-Gem and Redskin are highly susceptible. If you have a history of this disease in your orchard make sure to upgrade your spray program next year by starting a growth stage earlier with applying effective fungicides than what you used to do before (at least start from petal fall). 

2. Symptoms of bacterial spot of peach are visible on leaves: https://ufstonefruit.files.wordpress.com/2013/08/leaves.jpg  and on some locations of fruit (Fig. 1). The difficulty is discerning the symptoms of copper leaf injury on leaves from bacterial leaf spot on leaves. Luckily, here is a short article by Dr. Kari Peter at Penn State University explaining the differences: http://ipm.uconn.edu/documents/raw2/html/816.php?aid=816   Typical symptoms on leaves, which usually express before symptoms on fruit are shown in Figure 2. 

Figure 1. Early growing season bacterial spot infections on peach fruit visible as deep pits or cracks (Photo by Miller P. C. 2021)

Figure 2. Bacterial spot infections on peach leaves visible as angular spots (Photo by U. Mazzucchi, University of Bologna, Italy, Source: Bugwood.org)

The bacterial pathogen Xanthomonas arboricola pv. pruni causing peach spot of leaves and fruit overwinters in dark, gummy branch tips and twig cankers which develop from fall infections through leaf scars (copper products applied at leaf drop in fall will protect the leaf abscission scars from infections and will help with leaf-curl control; pruning removal of cankered or dark twigs during spring pruning can help reduce infection sources). Cankers develop in size the following spring releasing bacterial cells in the form of ooze droplets and are spread by rain and aerosol to young leaves, fruit and shoots. Spring with frequent rainfall with moderate temperatures and winds will favor the leaf and fruit infections. Symptoms on leaves are usually seen after bloom but infections occurred weeks before. Protection of leaves is necessary as the high amount of leaf infections will lead to more fruit infections. If warm and wet conditions continue longer infections after pit hardening are possible. Fruit infections take place after shuck split until pit hardening and require protection by alternating sprays of antibiotic oxytetracycline (FireLine, Mycoshield), Syllit plus captan, and low rate copper (however, copper can injure leaves and fruit under slow drying conditions). Keep in mind that oxytetracycline needs to be applied repeatedly when rainy and is mostly providing good control to merely suppressing the disease. However, oxytetracycline is the material least prone to causing phytotoxicity on peach and nectarines. Poor timing in delivery of this antibiotic and it's overuse can lead to loss of it's efficacy due to bacteria resistance. There is some evidence that the hydrochloride form of oxytetracycline (Fireline) is more effective than the calcium form Mycoshield. Do not apply either FireLine or Mycoshield on peaches or nectarines during 21 days before harvest. Syllit plus captan mixture is moderately effective  and can cause phytotoxicity if no rain occurs between applications (do not add foliar nutrients or surfactants when applying Syllit plus captan). In wet conditions, sites with strong winds and sandy soils allow bacteria to spread better and then enter the leaves and fruit because the sand particles hitting them create abrasions and nicks, a perfect entry points for this pathogen. The best control of bacterial spot is planting resistant cultivars (e.g. Biscoe, Blazing Star, Candor, Contender, Harrow Beauty, Harrow Diamond, PF1, PF12A, PF24-007, PF27A, Starfire; source: Shane B. 2011). Many other cultivars have fair, moderate to good resistance to this disease, but can get infected severely if weather/site conditions favor this disease and chemical means of protection are not applied on time or as frequently as rain periods dictate.