The season of abundance in the garden often coincides with the emergence of plant diseases, and 2024 is no exception, especially following the torrential rains associated with Hurricane Debby.
Two diseases in particular are worth keeping an eye on: potato blight, the first case of which has been reported in Montérégie, and cucurbit downy mildew.
According to two recent information bulletins from the Réseau d’avertissement phytosanitaire, a network of agronomists working for the Ministère de l’Agriculture, des pêcheries et de l’alimentation du Québec, potato blight is of particular concern due to its virulence and aggressiveness. Following infection, potato blight – a disease caused by the microorganism phytophtora infestans – can destroy potato and tomato plants in a matter of days.
In 2023, several outbreaks of potato blight occurred in different regions of Quebec, causing significant damage to growers. Humid conditions coupled with cool temperatures favour its development, and spores are easily transported by the wind over dozens of kilometers. This same blight was responsible for the Irish famine of 1846.
Precautions to be taken
As a preventive measure, it is advisable to ensure good aeration between plants (leaf removal, weeding, etc.), avoid watering plant foliage or watering in the evening, and plant only certified potatoes, not those found in grocery stores.
Once the disease, which is characterized by dark-brown spots on the fruit or stem, has been identified, it is best to destroy the plants over an area twice the size of the affected plants and dispose of them to reduce the risk of spread. Composting should be avoided.
Cucurbit downy mildew is caused by an entirely different disease, Pseudoperonospora cubensis, which only affects the cucumber family. Not to be confused with powdery mildew, it is highly virulent and can spread rapidly, as was the case across Quebec in 2023. This resulted in significant damage last year to fields of fresh cucumbers and embroidered melons, among others.
It is characterized by greenish-yellow spots that quickly turn brown, giving the leaves a parched appearance. If you suspect downy mildew, it’s advisable to harvest all the fruit and shred the residues with a lawnmower before burying them, to reduce the risk of propagation.
Fortunately, downy mildew does not survive Quebec winters, but it can be transmitted to some wild host plants including wild cucumbers. Prevention is therefore essential, notably by ensuring good aeration as described for potato blight, and by being extra vigilant.