GREENHOUSE CROPS
Avoid Geranium Bacterial Blight By John Hartman
Annual plants to be sold for spring sale and placement in the landscape are grown in many Kentucky greenhouses.  Geraniums are popular and growers are ordering cuttings or rooted cuttings for pot plant production later in the spring.  In past years, bacterial blight of geranium has been devastating in some geranium crops. Geranium Bacterial Blight.  This disease causes brown leaf streaks, leaf spotting, leaf wilt, whole plant wilt, and decay of the stem as the infection spreads from the vascular system to the pith and cortex.  In the diagnostic laboratory, geranium leaves with brown streaks along the veins or brown spots are typically examined for evidence of bacteria.  These leaf infections normally yield large numbers of bacterial cells of the causal agent, Xanthomonas campestris pv. pelargonii, which can be observed via microscopic examination. X. campestris pv. pelargonii can be harbored on plants overwintered in the greenhouse from the previous year’s crop.  Stock plants in the propagation greenhouse may be free of visible symptoms while harboring the bacteria as a latent infection.  In this way, bacterial blight can be introduced into the greenhouse on contaminated cuttings.  Once in the greenhouse, it can be spread easily from infected to healthy plants via splashing water and via contaminated hands pots, and tools. Managing bacterial blight.  Bacterial blight is extremely infectious and diseased plants cannot be cured.  To control bacterial blight of geraniums, use cultural practices that prevent the disease. Promptly remove and destroy diseased plants. Keep production areas clean by disinfesting work areas and tools.  The bacterial pathogen also may survive in potting media and soil and can be moved by shoes, shovels and hands. Use good growing practices such as proper fertilization, good plant spacing, and proper watering practices that will minimize wet foliage.  Avoid frequent handling of the plants. Purchase cuttings from propagators using culture-indexing programs to lessen the chance of bacterial blight from getting into the crop. Do not place culture indexed geraniums near non-culture indexed plants or near holdovers from a previous crop. Control insects such as whiteflies which could spread the infection. Grow geraniums from different suppliers in separate greenhouses; do not hang ivy geraniums, which are also susceptible to bacterial blight, above geranium crops. What is a culture-indexed cutting?  Bacterial blight can be transported into a geranium crop by an infested cutting.  The cutting may appear normal and healthy, but it contains tiny amounts of bacteria that later grow and cause disease in the contaminated plants and spread throughout the crop.  Growers reduce the likelihood of getting infested cutting material by purchasing “cultured” or “culture-indexed” cuttings.  These cultured cuttings are the end result of a complex and meticulous program to develop and produce pathogen-free geraniums. Long before a culture indexed cultivar comes on the market, a heat-treated or meristem-derived disease-free plant is selected.  Cuttings taken from this plant are taken to the laboratory where parts of the cutting are dissected and cultured or tested for bacteria and other pathogens.  The top part of the cutting is carefully rooted and potted in an isolation greenhouse.  If all tests are negative, the cutting is grown on to form a nucleus of plants that will yield more cuttings.  These, too, are run through the culture-indexing procedure to check for pathogens missed in the first assay.  Sometimes, just to be sure, a third round of culture-indexing is done. Cuttings that survive all the testing for pathogens become “mother” plants that are used to increase the number of plants of the disease-free cultivar.  From these cuttings, large groups of production plants are produced.  Growers who purchase culture-indexed material get plants from these production blocks.  Cuttings from the production blocks are now four generations away from the original culture-indexed plants.  If stringent precautions have been taken through these four generations, the cuttings should still be disease-free. However, production blocks are located in a different greenhouse - possibly even in a different country from the original nucleus material.  This is especially true in a global economy where high production costs can be reduced with low-cost labor.  Although the material may have been pathogen-free to begin with, it could become infested somewhere in the production process.  Growers will have best success with purchasing culture-indexed cuttings from the company that manages the original steps in the program (or a licensed propagator).  These companies have their reputations to uphold and take care not to allow their stock to become contaminated. Kentucky bedding plant producers also have their reputations to uphold and will want to invest in the culture-indexed plants so that their customers have the best quality geraniums possible.  Be aware that when these plants are being sold, these more expensive plants are often displayed at the outdoor retail garden centers side-by-side with less expensive and possibly diseased plants.  Where possible, growers need to observe how their geraniums are being marketed so that disease-free plants are not becoming contaminated. Figure 1 – Bacterial blight causing geranium leaf spot and blight. Figure 2 – Infected geranium plant with dead leaves and stem canker.
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NOTE: Trade names are used to simplify the information presented in this newsletter. No endorsement by the Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named.

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