Alternaria

After a prolonged cool and damp (but not wet) period, temperatures are about to rise dramatically. Combined with some frost damage and canopy stunting due to the long, cool period, this will greatly increase the risk of Alternaria leaf and stem blight. Hot and humid conditions are ideal conditions for infection and spread of the disease. The keys to good Alternaria control include:

1. Apply a rotation of effective products. Using products with multiple modes of action or tank mixing effective products with different mobility in the plant (if permitted on the label and compatible) can help with disease control and resistance management.
2. Ensure good spray coverage of all leaf surfaces and the stem. For more information, consult Spraying Ginseng on Sprayers101. The post rows, where coverage is often the poorest, are often the start of a disease outbreak. In 2-year-old gardens without a thick canopy, alternating the direction of travel of the sprayer can ensure areas that are difficult to reach may get at least some protection. Doing this in older gardens may cause damage to the canopy between the beds.
3. Reduce plant stress by ensuring adequate ventilation of heat (raise side shades, open up vents at the top of hills within the garden) and timely irrigation to avoid drought stress. Delhi has only had 8 mm of rain in the past four weeks, so soil moisture may be an issue if there are no thunderstorms tonight or tomorrow before the heat arrives.

Leaf Folders

The first leaf folders have reached the pupal stage (Figure 1). It usually takes about 3 or 4 weeks for adults to emerge once pupae are found, based on our experience rearing them out in previous years. We will then try to find eggs to determine if there is a second generation that is overlooked. Some caterpillars are still actively feeding (Figure 2). Insecticides will be less effective now that the majority of the caterpillars are in late development stages, and the active caterpillars tend to stay within the leaf rolls. White pupae of parasitic wasps are also commonly found within the leaf rolls (Figure 3). This is a good sign because it could mean that natural controls may eventually reduce this outbreak. In addition to causing reduced photosynthesis due to leaf feeding and rolling of leaves, leaf folds are a problem because they can also be refuges for fungal diseases, since fungicides cannot reach inside the fold.

Figure 1. A pupa of the zigzag Herpetogramma leaf folder.

Figure 2. Multiple leaf folds within a ginseng canopy.

Figure 3. A parasitic wasp pupa next to a dead leaf folder caterpillar.

Other Diseases

There are numerous unusual symptoms in ginseng gardens this spring, probably due to the prolonged cool and wet period in May and repeated mild frosts. The most common is wilting and decay of the lower stem within the straw. Rhizoctonia is likely involved in many of these symptoms, but there are wilting plants with no sign of Rhizoctonia symptoms. Verticillium and various minor root pathogens could also be involved. Laboratory diagnosis may be required to determine the cause of these symptoms. Contact me if you are dealing with unusual symptoms, and the ginseng research team may be able to help.

Interpreting Laboratory Diagnostic Reports

For growers and consultants who submit disease samples to the laboratory, it is very important to know how to interpret the results of those tests. The presence of an organism in the diseased sample does not mean it is the cause. Interpreting a test wrong could lead to the wrong pesticide being applied or pesticides being applied when they are not warranted. Click here for more information on how to interpret a diagnostic report.