We have had about 36 straight hours of leaf wetness over the past two days. This combined with heavy downpours and moderate temperatures both day and night are perfect conditions for the development of foliar Phytophthora. Re-application of foliar fungicides may be required.
This is the time of year that tops begin to show stresses related to root diseases and other issues. Here is a summary of different types of issues that are showing up:
Some plants in older gardens are showing top senescence even though the stem and root look relatively healthy. In these cases the stem appears to be a little stunted, fattened and slightly twisted. It is likely that this is related to poor stem development when it was enlarging in early May. The prolonged cold period in May could have affected stem and leaf development even if frost didn’t occur. Many fields are showing shorter stems than they would normally be, without the typically stem splitting or kinking that are associated with a frost event.
Another common symptom is severe stunting and wilting more widespread in sections of fields. This is most common in 2-year old gardens. Pythium seems to be the most prevalent issue in these areas. Pythium root rot often results in proliferation of lateral roots and fattening of the root tips. The damage to the root tips results in poor water uptake and can lead to early senescence or wilting during the heat of the day. There are also plants lacking any lateral roots with the tap root turning wrinkled and pliable, which is probably due to severe disease. Maestro/Captan is registered for suppression of Pythium root rot. Rootshield WP is also registered for suppression of both Pythium and Fusarium. Some products applied for Phytophthora control (e.g. Reason and Ridomil Gold) may also provide suppression of disease.
Random and scattered plants in 2-year old gardens are also showing reddening and wilting of leaves (Figure 1). This symptom is very common and is associated with roots that have a mushy interior with an intact skin. Sometimes if the skin is squeezed, the interior comes out like toothpaste. It is often associated with Rhizopus, a common soil fungus, but the actual cause has not been fully determined. Pythium and Fusarium are also frequently found associated with these symptoms. The pattern in the field would suggest a seed-related issue. The disease does not seem to spread in the field and the loss of a few scattered plants within an otherwise healthy canopy should have very little impact on overall yield.
Figure 1. Scattered plants collapsing due to root disease in 2-year old gardens. Roots often have just a shell remaining in the ground.