Ginseng growers continue to deal with Alternaria issues in frost damaged fields. If you have applied fungicides on a tight rotation for the past month and are still not getting control then one of the following is probably occurring:
- Plants were too damaged by frost to defend against invasion by Alternaria. If this is the case, then plants would have died anyway, and the half dead tops are being invaded by Alternaria and other common fungi. Fungicides may reduce inoculum for next year but will not keep tops alive under this scenario.
- You are not getting good coverage of all plant surfaces with your pest control products. The Sprayer Workshop held on July 2 highlighted the importance of ensuring you are getting good coverage by putting water sensitive papers in the garden at various points inside the canopy and seeing whether the spray is hitting the target. It also highlighted the importance of drop arms in all trenches of the garden, ensuring all nozzles across the boom are putting out the same volume, and ensuring that the boom is at an appropriate height above the canopy so the spray from each nozzle overlaps. If, for example, your spray does not reach within the canopy in the post row, this will be a reservoir for inoculum for new infections. It is also important to apply the right water volume for the product being applied. More volume is not always better if the target is the leaves.
- You are not using the appropriate rotation of fungicides. Applying the same fungicide over and over can lead to resistance issues. It is important to constantly switch to new fungicides from different fungicide groups throughout the season. Application of a more expensive but also more targeted fungicides may be necessary to get the disease under control.
- Plants may be stressed by fertility issues, other diseases or adverse weather. A stressed plant will be much more likely to develop Alternaria. The heavy rains this past month have resulted in excessive water falling from the drip-lines. This has led to drip-line chlorosis (yellowing), which can then make plants more susceptible to disease. Additional applications of fertilizer may be necessary after heavy rains to replace what may have leached away from the root zone.
