Pseudomonas Bacterial Spot

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Taylor Mauch, Lina Rodriguez-Salamanca, and Ajay Nair
ISU Department of Horticulture and Plant and Insect Diagnostic Clinic

Pseudomonas Bacterial Spot is a disease that causes spots on leaves and yellowing of leaves that eventually can drop off.  As many other foliar disease, defoliation increase the risk for sun scald of fruit and lessens overall plant productivity. Scab-like lesions can occur on the fruit causing un-marketability through poor appearance. This disease is spread through wet conditions so transmission through the field is worse under times of high precipitation. Identifying this disease can be done under a laboratory and diagnostic setting such as the Plant and Insect Diagnostic Clinic at Iowa State University.

Figure 1 Figure 2 Figure 3
Figure 1. Whole-plant appearance Figure 2. Early-stage appearance Figure 3. More developed appearance

Management options include the use of disease-free seed, greenhouse and field sanitation, crop rotation, weed control, selecting resistant varieties, applying labeled copper fungicides and rotate mode of action (FRAC group) to avoid copper resistant bacterial populations. Fungicide sprays are most effective to protect healthy plants in early stages of the disease. Before planting, greenhouse practices (irrigation) should focus reduce spread of disease. Proper washing and sanitation of seed trays if reusing them and sanitization of tools helps to reduce the possibility of transmission. If the seed is already infested hot-water treatments can be carefully done (though this can be challenging as high temperatures can affect germination rate) or bleach concentration suitable for seed treatment can be used, though it may be less effective as it only treats the seed surface. If the seed-treatment option is taken, it should be done within a few weeks of planting with the best option occurring right beforehand.

 After planting, irrigation method is also a consideration. Since this disease spreads through water splash, drip irrigation rather than overhead irrigation is recommended. If taking applying copper products, treatments can be done every 7-10 days starting with a copper fungicide. Under wet conditions, use the 7-day spray schedule with the 10-day spray schedule occurring under drier conditions. Scout regularly and remove disease-infested plants to prevent possibility of transmission. Weed control is of importance as weeds can harbor the bacterial pathogen Pseudomonas. Taking care to remove weeds helps to reduce possibility of transmission to harvestable plants of market value. Crop rotation can help reduce pathogen population in the soil. These should be planned on a 3-year or more schedule. Inducing rapid decomposition within the field helps to reduce populations of pathogens in host plants. This can be done by tilling or disking immediately after harvest.

Date of Publication: 
May, 2022