Chapter 23

Greenhouse Disease Management

Bacterial Diseases of Greenhouse Plants

Plant pathogenic bacteria generally survive in infected plants, in debris from infected plants, and in a few cases, in infested soil. Most require a wound or natural opening in the plant to gain entry and require warm, moist conditions in order to cause disease. Depending on the species of bacteria involved and the tissue infected, they release enzymes that degrade cell walls, toxins that damage cell membranes, growth regulators that disrupt normal plant growth, and complex sugars that plug water conducting vessels. Bacteria reproduce very rapidly. They are splashed easily from the soil to the leaves and from leaf to leaf by overhead irrigation. They are also easily moved from soil or debris when a worker handles such material and then handles the live plant. The most important means of avoiding crop losses caused by bacteria is to purchase plants that have been shown to be free of such pathogens by the process of culture indexing.

Bacterial Leaf Spots

Bacterial leaf spots are caused by Erwinia, Xanthomonas, and Pseudomonas species of bacteria. Bacteria infect foliage, fruit, and stems, but require openings such as lesions to get inside the plant because they are relatively weak pathogens. Commonly, insects create lesions from feeding on plants. Fungus gnat larvae can spread soft rot bacteria. The pathogen itself is seedborne, which can then spread to other nearby plants after the seedling begins to grow through splashing water and overhead irrigation.

Symptoms

Bacterial leaf spot on plants may manifest in several different ways (See Figure 23.11). 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

Promote leaf drying by managing irrigation and air circulation. Eliminate overhead irrigation and exposure to rainfall when possible. Splashing water moves bacteria and allows them to infect new leaves. Destroy infected plants, being careful to avoid contact with other plants. Remove all plant debris, and keep tools and benches free of unsterilized soil which may harbor soft rot bacteria.

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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