2025 Late Blight Update

With high late blight pressure the last two seasons, there may be enough inoculum around for an outbreak. Approach this season with a mentality of “when” late blight shows up and not “if”.

If you have or know of a backyard or local community garden, monitor tomatoes/potatoes for late blight symptoms and destroy infected plants. These can act as reservoirs and inoculum sources for commercial fields. Late blight can spread and leap frog around the province using these untreated gardens.

If you grow potatoes or tomatoes, here are some important reminders for late blight management:

Tomato Late Blight Management Reminders and Spore Trap Results

This is a community disease and if you see late blight in your field, please notify OMAFA staff. If late blight is found in your field or a neighbouring field, fungicides with specific late blight activity should be added to your fungicide program to prevent significant crop losses. Always be sure to read every product label carefully before each application.

The Spornado Spore trap project in tomatoes continues for 2025.

Forecasting models using local weather data suggest that Kent County is at high risk for late blight at this time.

Results: spores for late blight were captured around Harrow, Port Lambton, Rondeau, Cedar Springs, Union Station and Simcoe.

Potato Late Blight Management Reminders

Volunteer management will be very important this year. We are seeing high volunteer survival rates over the winter in rotational crops. Control those potential inoculum sources either with herbicides or tillage or even rogue them out. While on the topic of killing potatoes, destroy all cull piles.

We have seen lots of vigorous potato growth this past week and lots of new, unprotected leaves. Ensure your fungicide program is protecting this new growth. Two products that were key in successful late blight fungicides programs the last two seasons are Orondis Ultra and phosphorous acid (P-acid) products like Phostrol/Confine Extra/Rampart. These should be applied early, prior to late blight infection.

Addition note on phosphorous acid (P-acid) products like Phostrol/Confine Extra/Rampart; These products work in two ways, one is directly on late blight by suppressing germination and growth but the other way is to activate the plant’s natural defense response. If you use these products, make sure they’re applied early and often.

For a full list of products registered for late blight and relevant label information like rate, PHI, max apps, etc. please visit Vegetable crop protection – Potatoes – Late blight.

With this stretch of hot weather we’re seeing lots of heat stress/burning/air pollution symptoms. Sometimes those symptoms can be confused with late blight lesions so it’s important to distinguish them.

• 3 key difference between air pollution/heat stress symptoms vs. late blight:
  • Main veins will remain green
  • No white fuzzy mycelial growth on the underside of the leaves
  • No yellow halo

Want a refresher on what late blight looks like? See the potato late blight entry in Ontario CropIPM.

Suspect Late Blight?

If you suspect late blight in your crop, please reach out to the appropriate OMAFA specialist to confirm the diagnosis.

Tomato – Amanda Tracey (amanda.tracey@ontario.ca, 519-350-7134)

Potato – Dennis Van Dyk (dennis.vandyk@ontario.ca, 519-766-5337)