Plants in most gardens are beginning to show signs of stress including brown leaf edges and red discolouration of leaves. These symptoms are probably due to a combination of heat stress, moisture stress, and early senescence. Due to warm conditions this spring, many plants are about 3 weeks ahead of schedule this year. As a result, plants are at the same stage they would normally be in early to mid-August, when early signs of senescence are more common. Alternaria is the main disease that infects and spreads under hot and dry conditions. Symptoms of root disease often show up under dry conditions, but most of the damage is usually caused earlier in the season under moist conditions.
Pythium has shown up in many gardens. Pythium causes the tips of the fibrous roots to appear water soaked and often enlarged. As a result, plants are unable to take up as much water and the top can wilt, beginning in the heat of the day. Once moisture stress becomes severe, the plant may respond by aborting the top to preserve the root. The top may become loose and easily pulled out of the ground, and will collapse and senesce rapidly. This will stop growth of the root, but often the plant will come back the following season. Pythium rarely kills the plant entirely, but may cause more spider-shaped roots as new fibrous roots are formed to replace those damaged by the disease. Pythium is a water mould that thrives under warm and moist conditions. It can be more of a problem in the heat of the summer than Phytophthora, which is also a water mould. Application of Ridomil for Phytophthora will provide some control of Pythium.
Dew periods are beginning to lengthen and leaves are remaining wet for a longer period overnight and into the morning. Combined with periodic showers, this will increase the risk of foliar disease. Any heavy rain from thunderstorms can immediately increase the risk of both foliar and root diseases, especially under the drip lines. Growers should continue to monitor for all diseases, even if low levels of disease have been found for an extended period. As of the morning of July 26, there has been insufficient rain in many areas to saturate the entire root zone if the field was not recently irrigated. While more rain is forecast, it is important to continue monitoring soil moisture levels and ensure timely irrigation to avoid moisture stress.
Dry Weather Information:
Visit the OMAFRA website for links to information on dry weather and low water conditions. Go to www.ontario.ca/z832.