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Pennsylvania Flower Growers #203


One of the most common diseases of geraniums is Botrytis blight. It is caused by the fungus, Botrytis cinerea, the common gray mold which attacks many floricultural crops. The fungus grows best at a temperature range of from 45 to 60°F. Its spores are developed under cool, moist conditions and to invade a plant it requires free moisture on the tissue it is at tacking for 8 to 12 hours. It may attack dead tissue or live tissue that has been injured or weakened or has become aged. A severe outbreak of the disease may start as follows. Stubs on the stock plants left after the cuttings have been taken are invaded by Botrytis. Under the cool conditions and high humidity found down under the thick foliage of the stock plants, a mass of spores is produced on the surface of the stubs. Viewed under the microscope it is seen that the spores are borne on branches of the fungus threads or mycelium like bunches of grapes Fig. 1. This cluster ing of the spores is the basis for the name Botrytis which is derived from the Greek word 'botrys' mean ing cluster of grapes. The individual spores are oval in shape and are very light. With changes in relative humidity around the plant the spores are released and are carried in the air currents to the flowers which may become infected if water stays on them for 8 to 12 hours. Regardless of whether they are infected or not, the aging petals fall to the leaves and if moisture is present for a long enough period, the leaves become infected. In both eases, infection of the flowers and of the leaves, more spores are produced to add to the thousands formed on the stubs and on bits of dead tissue of leaves, leaf scales and petals on the soil under the plants. Many of the spores are lodged on the hairs on the stems Fig. 2 and when more cuttings are taken and are stuck in the rooting medium, the moist conditions are ideal for infection and severe losses of the cuttings results Fig. 3.

Keywords: symptoms Botrytis blight Botrytis cinerea GERANIUM RUST Preventive measures

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