Scout the tomatoes, dodge the heat dome, bag the bad leaves before Sunday.
Late blight on Portland tomatoes is the disease I lose sleep over every June, and this week the conditions are unusually loaded. We just took a 0.6 inch soaking on Monday, and by Sunday we are forecast to hit 98°F — that swing from wet leaves to humid heat is exactly how blight, powdery mildew, and spider mites all get their foothold at once. I'm walking the beds every morning with coffee in one hand and a paper bag in the other, and I'd urge you to do the same.
This Week's Action List
- Inspect every tomato plant top to bottom before Friday. Late blight starts as olive to brown water soaked spots on lower leaves, often with a faint white fuzz on the underside in morning dew. If you see it, pull the entire plant, seal it in a contractor bag, and put it in the trash — never the compost or yard debris bin. Spores travel on wind for miles.
- Get ahead of powdery mildew on zucchini, cucumbers, and squash this week. Thin out crowded interior leaves so air moves through the vines, and spray a preventive mix of one tablespoon potassium bicarbonate plus a half teaspoon horticultural oil per gallon of water on both leaf surfaces. Spray in the cool of early morning, never during Saturday or Sunday's heat.
- With Sunday's 98°F forecast, spider mites will multiply overnight on beans, eggplant, and tomatoes. Hold a sheet of white paper under a suspicious leaf and tap — if you see specks crawling, hose the undersides of leaves with a strong jet of water in the morning, then follow up with insecticidal soap or neem oil at dusk for three evenings in a row.
- Walk your apples and pears for fire blight while you're out there. Look for branch tips that appear scorched, with leaves still clinging and a shepherd's crook bend at the end. Cut a full 12 inches below visible damage, dip your pruners in a 10 percent bleach solution between every cut, and bag the prunings. Do not prune fire blight in rain — wait for Thursday or Friday's dry window.
- Mulch tomatoes with two inches of straw or dry grass clippings before the heat arrives. A clean mulch layer keeps rain and irrigation from splashing blight spores from the soil up onto lower leaves, and it also buffers the root zone during Sunday's spike. While you're at it, prune off any tomato leaves touching the ground.
Frequently Asked Questions
How do I tell late blight from early blight on my Portland tomatoes?
Early blight starts on the oldest lower leaves as small brown spots with concentric rings, like a target, and moves slowly up the plant. Late blight hits anywhere on the plant as larger olive or brown water soaked patches, often with a pale fuzz on the leaf underside, and can collapse a whole plant in under a week. When in doubt in June, treat it as late blight and remove the plant — the cost of being wrong is your entire bed.
Can I save a tomato plant that has late blight, or do I have to pull it?
I've never successfully saved one, and I've tried. Once those water soaked lesions appear, the spores are already releasing into the air and onto neighboring plants with every rain splash or overhead watering. Bag the plant the same day you spot it, sanitize your pruners and gloves, and resist the urge to compost any part of it.