This season, experts are warning Prairie gardeners—especially those in Calgary, Alberta—that late blight may pose a serious threat to your vegetable crops. If you notice any infected tomatoes, potatoes, or other Solanaceae family plants, it’s crucial to remove and destroy these plants immediately to prevent the disease from spreading further in your garden.
What Is Late Blight?
Late blight, caused by the pathogen Phytophthora infestans (the “plant destroyer”), is infamous for causing the Irish Potato Famine in the 1800s. It’s a fungal-like disease that affects members of the Solanaceae family—including tomatoes, potatoes, peppers, and eggplants—causing devastating damage to leaves, stems, and fruits.
Historically, it originated in Mexico and the Andes, then spread rapidly across Europe and North America, leading to widespread crop losses whenever environmental conditions favor its development.
Late Blight on the Prairies? Yes – and Recent Weather Patterns Are Conducive
Although late blight is more common along Canada’s East and West Coasts, recent weather conditions in Alberta, Saskatchewan, and Manitoba have created an environment highly favorable for the disease. Warm temperatures of 15-20°C (59-68°F), combined with frequent rain and heavy dew, can lead to explosive growth of late blight, potentially wiping out potato and tomato crops in the area.
How Does Late Blight Show Up?
- On Potatoes: Look for black or dark brown lesions that start at the edges of the leaves and move inward. The leaves may yellow and wilt, and stems can turn black and droop. If spores wash into the soil, tubers can rot from within, often without obvious external signs.
- On Tomatoes & Peppers: Early signs include water-soaked spots on the “shoulders” of the fruit, with reddish-brown lesions and a yellowish halo around them. On the undersides of leaves, you may notice white, cottony growth, especially during damp mornings. As the disease advances, infected stems and petioles turn black, weakening the plant and causing the leaves to flag and fall.
What to Do if You Suspect Late Blight?
- Remove Infected Plants Immediately: Carefully pull out and destroy (do NOT compost) any infected potatoes, tomatoes, peppers, or eggplants. Bag them tightly and dispose of them away from your garden beds.
- Monitor Nearby Plants: Keep a close watch for early symptoms on healthy plants. Act quickly if you notice any signs of infection.
- Manage Storage Carefully: Avoid storing diseased tubers—late blight can survive on infected potatoes over winter and infect new crops. Use only healthy seed potatoes for planting next season.
Preventative Measures for Calgary Gardeners
While no potato variety is completely resistant to late blight, some cultivars offer greater resilience. Consider planting:
- Russet-type potatoes (which tend to be more resistant than red varieties)
- Resistant tomato varieties like Defiant, Legend, or grape types such as Santa and Juliet (although their resistance on the Prairies has not been fully tested)
Spraying & Treatments:
- Bacillus subtilis (Serenade): An organic, naturally occurring bacteria that feeds on fungi and helps suppress late blight if applied early and consistently—about every ten days after plants reach the two-leaf stage.
- Copper Sprays: Can be effective but need to be reapplied following rain or overhead irrigation.
Weather & Future Risks
Heavy winter snowfalls have created conditions conducive to late blight development this season. Vigilance is essential. Start preventative sprays early and continue monitoring your plants, especially during rainy or humid periods, to catch any signs of disease before it spreads.
Final Tips for Calgary Gardeners
- Avoid Over-Fertilizing: Excess nitrogen encourages lush, dense foliage, which is more susceptible to blight.
- Prune & Stake: Improve airflow around tomato and potato plants to reduce humidity and disease risk.
- Choose Resistant Varieties: While no plants are fully immune, certain cultivars are better equipped to withstand late blight.
Stay vigilant, act early, and keep your Calgary garden healthy! Late blight is a formidable foe, but with the right precautions, you can protect your crops and enjoy a bountiful harvest.
