August and September bring humidity and cooler morning temperatures. This combination leads to a lot of morning dew and daytime humidity, the perfect situation for fungal diseases such as powdery mildew and blight. These mostly affect long-lasting, broad-leafed plants such as tomatoes, potatoes, squash, cucumber, and zucchini.
While there is little chance of preventing these diseases in an outdoor garden, you can reduce and defer them through good growing practices. The goal is to buy yourself more time to grow and harvest your vegetables. Here are a few good practices to follow:
- Remove most lower leaves on fruiting plants – in particular, any leaves below the lowest fruit. In addition, remove any suckers and leaves that are crowding the middle. Dense areas can create pockets of stale air and harbour fungal spores. Of course, make sure there is still sufficient foliage to photosynthesize.
- Weed around susceptible plants to improve sunlight and increase airflow. Remove any nearby piles of waste that can hold moisture or host blight/mildew spores.
- Use row cover on young squash and zucchini plants to reduce the effects of morning dew. Remove the cover during the day for air flow and to allow pollinators as the plants flower – but remember to put it back on at night.
- Optimize plant health. Healthy plants are better able to withstand blight and powdery mildew so keep your plants well fertilized and evenly watered at this time of year.
- Once you see any signs of these diseases, use a mixture of 1 teaspoon baking soda, 1-2 drops of dish detergent, and 1 liter of water in a spray bottle and spray all affected areas of the plant every couple of days.