With the hot and dry conditions over the past two weeks, Alternaria problems have increased in ginseng gardens over the past week. Much of the damage is currently along the stem with tan, brittle lesions covered in black conidia (spores). Tops will not recover from this type of damage. Once infection is noted in a garden, calendar sprays of a rotation of registered fungicides are recommended. Adjust the interval between sprays based on weather conditions. Alternaria prefers warm temperatures with high humidity, but does not require an extended wet period for infection.
The wet weather expected over the weekend will increase the risk of both Botrytis and foliar Phytophthora. Foliar Phytophthora has been reported in areas with root rot from last year. Conditions so far this year have not been favourable for the spread of foliar Phytophthora, but that can change rapidly if leaves remain wet for an extended period. An initial wet period of about 12 hours or more can result in infection of leaves followed by release of sporangia (spores) several days or up to a week later. The risk is higher under cool conditions. If another extended wet period occurs at that time, the disease can spread rapidly and affect all areas of the garden. Ensure plants are protected from foliar Phytophthora throughout the next few weeks. Maestro has received emergency use registration for the control of Phytophthora for the 2012 growing season. Botrytis may still be a problem on plant tissues damaged by frost. Wet conditions will also promote spread of this disease.
Leaf roller damage is also showing up in ginseng gardens. Because leaf rollers do not always require a control, it is essential to base the application of insecticides on a scouting program. Leaf rollers are best controlled at an early stage before symptoms become obvious from a distance. Garden walkers should be encouraged to examine leaves closely in different areas of the garden and look for the small caterpillars or rolled up leaf edges. Dipel 2X DF is the only product registered for the control of leaf rollers in ginseng.