Greenhouse Disease Management
Greenhouse Bacterial Diseases
Bacteria are single-celled microorganisms. Bacteria are mostly dependent on outside agents to get them from plant to plant. Bacteria do not penetrate directly into plant tissue but need to enter through wounds or natural plant openings. Wounds can result from damage by insects, other pathogens, and tools during operations such as pruning and picking. With few exceptions, plant pathogenic bacteria cause disease by colonizing the internal tissues of plants, thereby interrupting normal growth and function. Bacteria grow between plant cells as opposed to inside host cells, like viruses do. As a result, the way that they cause disease and the symptoms that they trigger are quite varied. Bacteria cause a variety of symptoms including leaf spot, bud rot, canker, vascular wilt, soft rot, and galls. Symptoms caused by bacteria are often indistinguishable from those caused by fungi. Bacteria that colonize the vascular system are systemic, they are transported relatively rapidly throughout the vascular system. Bacterial diseases found that infect greenhouse plants are difficult to control other than through prevention, sanitation, and removal of infect plants. Disinfectants can be used to sanitize greenhouse tools, benches, pots to provide some protection.
Bacterial Leaf Spots
Bacterial leaf spots are caused by Erwinia, Xanthomonas, and Pseudomonas species of bacteria. Bacterial leaf spot diseases are becoming more common, particularly in plug production where mist irrigation is often used. Bacterial soft rot is a common problem with poinsettia cuttings, often causing significant losses. Bacterial leaf spot diseases are common on zinnia and chrysanthemum
Symptoms
Bacterial leaf spot on plants may manifest in several different ways (See Figure 28.12). Symptoms of bacterial leaf spot may include black edged lesions, brown spots with yellow halos or just light and dark areas on the foliage. Spots are irregular and measure between 3/16 and ½ inch wide
Cultural Management Strategies
Ensure seed and transplants are disease-free. Minimize leaf wetness in the greenhouse. Water plants when the leaves will dry quickly (i.e., in the morning and early afternoon). Avoid watering plants just before dusk to minimize leaf wetness overnight. Minimize handling plants, particularly when wet.
Chemical Management Strategies
Bacteria can rapidly develop resistance to many active ingredients (especially copper and antibiotics). It is important to rotate between bactericides of different Fungicide Resistance Action Committee (FRAC) group numbers. For controlling Xanthomonas or Pseudomonas alternate a copper-containing product (M1–Camelot O, CuPro) with one containing streptomycin sulfate (antibiotic–AgriStrep) or Bacillus subtilis (microbial– Cease, Triathlon.
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