The persistent wet and cool conditions have resulted in very high risk of foliar Phytophthora. After the wet conditions last fall and this spring, there is a high amount of inoculum of Phytophthora in low areas in most gardens. When there is free water or saturated soils, the swimming spores of Phytophthora can be easily splashed onto the leaves, which results in a foliar infection if the leaf remains wet for a few hours. After a week or so of development in the leaves, sporangia of the fungus, which are pouches full of swimming zoospores, are formed daily on the outside of the leaves and are easily blown on the wind. When these sporangia land on a leaf surface, they just need a few hours of wetness to infect. Spores that wash into the soil from these leaves can result in new root infections even in areas of the garden that are not normally prone to the disease.
The frequent rains this month combined with days of prolonged leaf wetness this week has likely resulted in some foliar infections in many ginseng fields. Any additional wetness periods such as the rain forecast for this weekend could greatly increase the amount of disease if leaves are not protected with fungicides.
It is essential at this time to protect leaves from Phytophthora. This requires a lower volume spray than what would be used for controlling root disease similar to what is used for Alternaria control, and this should not be followed by a rainfall so it can dry onto leaf surfaces. The use of drop nozzles when spraying in older gardens is important to ensure good stem coverage. Spraying for foliar Phytophthora when the risk is this high should be considered a different application than what is used for root disease.
Control also requires the use of targeted products with some curative action to halt the spread of any current infections. While products like Aliette and Phostrol can be good options when the risk of disease is low to moderate for general protection, they will not control the disease when foliar infection is already present in the leaf. While xylem-mobile products like Ridomil can be taken up by the roots and moved to the leaves, this may not provide enough control of foliar disease under these pressures. Also, resistance to Ridomil has been demonstrated in the past in Ontario Phytophthora cactorum populations. This means that Ridomil may be ineffective in some populations of the pathogen. OGGA will be testing Ridomil against different Phytophthora populations in the lab this summer to determine the extent of resistance.
This week we had confirmation of Phytophthora infection of the lower stem within the straw, likely because of spores washing down the stem from the leaves. Stems arched over due to softening of the lower stem just like Rhizoctonia but without the rusty discolouration of the crown. Here is a simple technique provided by Amy Shi at OGGA for determining if you have Rhizoctonia or Phytophthora at the crown (refer to Figure 1):
- Gently rinse the roots under clean water to remove any soil from the surface.
- Place the root (or just the upper portions of a large root) and the lower stem into a sealed container on top of a folded moist paper towel.
- Stick a wooden toothpick into the top/crown of the root
- Seal the container and reopen after 12 hours (not any longer).
Figure 1. Demonstration of the toothpick technique for testing for Rhizoctonia. Within 12 hours, Rhizoctonia, if present, would grow up the toothpick once the container is sealed.
If there is a white, fluffy growth up the toothpick when you reopen the container, then it is likely Rhizoctonia. If nothing grows up the toothpick and the root becomes more water soaked and develops a sour odour over time, then Phytophthora is more likely. A sample may need to be sent to a diagnostic lab if neither occurs. Note: if you wait too long to reopen the container, then other organisms can grow up the toothpick and the test will not work.
Don’t forget about monitoring and management of other pests. Slugs and insect pests such as cutworms and leafrollers are already present and will be on the rise in the next few weeks. If at all possible, avoid having workers or machinery move through wet foliage or saturated soil in trenches as this can result in movement of Phytophthora inoculum throughout gardens.