By M. Nampijja, S. Crane, L. du Toit, and C. Ocamb
Cause Pseudomonas syringae pv. aptata causes bacterial leaf spot on both vegetable and seed crops of Swiss chard. Some strains of P. syringae pv. aptata can infect both beet and Swiss chard, while other strains can only infect beet. Strains of P. syringae pv. aptata have also been shown to infect plants outside of the Chenopodiaceae (Amaranthaceae), including cantaloupe, pumpkin, squash, bean, eggplant, lettuce, and pepper, as well as some weed species. The full host range of this pathogen is still being determined.
The pathogen can be seedborne as well in infected plant residues, and infection can occur at any stage of growth, from the cotyledons of seedlings (usually as a result of seed transmission after planting infected seed) to mature fresh market, processing, or seed crops. Infections in the field can be spread by splashing rain or irrigation water, and on contaminated tools or equipment. Cool, moist conditions favor bacterial leaf spot, including rainy weather, excessive irrigation, or extended periods of dews. Infection occurs at temperatures above 36°F and below 95°F, with optimal temperatures of 77°F to 86°F. The bacteria enter plant tissue through natural openings such as stomata and hydathodes (latter are small openings where the leaf veins reach the margin of the leaf), or wounds from hail, windstorms, insect feeding injury, and mechanical damage. The disease has become increasingly problematic in baby leaf red beet and Swiss chard production because overhead irrigation, dense seeding rates (2 to 4 million seeds/acre), and the potential presence of the pathogen on seed create highly conducive conditions for bacterial leaf spot.
Symptoms Pseudomonas syringae pv. aptata causes irregular-shaped to circular leaf spots, each measuring 0.20 to .25 inch in diameter. Water-soaked lesions containing dead tissue can appear on the margins of leaves or anywhere across the leaf surface and on cotyledons. Lesions can range in color from tan-to-dark brown or even black under very wet conditions, and usually develop a narrow, dark-brown-to-black margin. Lesions may also develop on seed stalks in seed crops. If lesions develop on young leaves or seed stalks that continue to expand around the dead tissue, this can cause the stems and leaf tissue to distort or bend. Bacterial leaf spot symptoms can occur with symptoms of other foliar diseases such as Phoma leaf spot or Cercospora leaf spot.
Cultural control
- Crop rotation with non-host plants of P. syringae pv. aptata.
- Manage weeds effectively to limit the risk of weeds serving as reservoirs of inoculum.
- Plant seed lots that have tested negative for P. syringae pv. aptata or seed lots that have been treated to eliminate the pathogen.
- There are no cultivars with complete resistance to bacterial leaf spot but there is variation in susceptibility among culti-vars. Efforts are in progress to breed for improved resistance to bacterial leaf spot in beet and Swiss chard.
- If possible, use drip or furrow irrigation to limit splash dispersal of the pathogen.
- If overhead irrigation is used, minimize the duration the crop canopy remains wet, e.g., by irrigating in the morning so the canopy is not wet through the night, or by irrigating longer but less frequently.
- Contaminated irrigation water, particularly surface water, can be a reservoir of inoculum. Irrigation water can be tested for P. syringae pv. aptata. Infected sources of water may need to be treated.
- Bury crop residues in the soil after harvest as infected crop residues break down more rapidly when incorporated into the soil than when left on the soil surface.
- In areas of biennial seed production, avoid planting a crop in close proximity to a seed crop in the second year of production, to reduce the risk of the pathogen spreading between fields. Similarly, sequential plantings within fields of fresh market, root, or processing crops increase the risk of spread of the pathogen.
Chemical control There are no systemic, highly effective bactericides for control of bacterial leaf spot.
- Copper formulations (Group M1) offer limited control. Numerous copper products are registered for use but must be used preventatively. They do not eradicate bacterial leaf spot once the disease has developed but can slow progress of the disease.
- Badge SC (copper hydroxide + copper oxychloride) at 0.5 to 2.5 pints/A on 7- to 14-day intervals. Preharvest interval is 0 days. 48-hr reentry.
- Oxidate 5.0 (hydrogen peroxide + peroxyacetic acid) is registered for other diseases on this crop at 1:800 to 1:500 dilution on 5- to 10-day intervals for preventative application; begin sprays early in season before disease onset. For curative applications, use at 1:256 dilution on 3- to 10-day intervals; apply at first sign/symptom of disease.
Biological control Although a number of biological control products are registered for use for control of bacterial diseases, the efficacy of these products for control of bacterial leaf spot is unknown. Many have not proven effective.
- Double Nickel LC (Group 44) at 0.5 to 4.5 pints/A on a 3- to 10-day interval. 4-hr reentry.
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
References Franc, G.D., Harveson, R.M., Kerr, E.D., and Jacobsen, B.J. 2001. Disease management. Pages 131-160 in: Sugarbeet Production Guide. R.G. Wilson, S. A. Smith, and S. D. Miller (eds.) Univ. Nebraska Coop. Ext. EC01-156.
Harveson, R.M., Hanson, L.E., and Hein, G.L. 2009. Compendium of Beet Diseases and Pests, 2nd Ed. St, Paul, MN: APS Press.
Nikolić, I., Stanković, S., Dimkić, I., Berić, T., Stojšin, V., Janse, J., and Popović, T. 2018. Genetic diversity and pathogenicity of Pseudomonas syringae pv. aptata isolated from sugar beet. Plant Pathology 67:1194-1207.
Rudolph, K. P. 1997. Pseudomonas syringae pathovars and related pathogens. Kluwer Academic Publishers.
Sedighian, N., Shams-Bakhsh M., Osdaghi, E., and Khodaygan P. 2014. Etiology and host range of bacterial leaf blight and necrosis of squash and muskmelon in Iran. Plant Pathology 96:507-514.