✨ Hot afternoons, cool nights, and a garden full of quiet infections waiting for a slip in vigilance.
Portland garden diseases in July rarely announce themselves loudly, and this week's forecast of 77 to 88°F days with nights dipping to the low 50s is exactly the split that lets trouble sneak in. I'm walking the beds every morning with coffee in hand, because the dry heat masks fungal problems on cane berries, dahlias, and stone fruit even as it drives spider mites and bacterial issues elsewhere. Here's what I'm watching, and what you should be checking on your own patch this week.
This Week's Action List
- 1
Scout raspberries and blackberries for cane blight and spur blight now. Look for purple to brown lesions ringing the cane at leaf nodes, and cut affected canes to the ground with bypass pruners disinfected in 70 percent isopropyl alcohol between cuts. Bag the prunings, do not compost them, because the fungus overwinters on debris.
- 2
Check dahlia foliage for the first signs of viral mosaic and powdery mildew. Mosaic shows as pale green mottling and puckered leaves, and infected plants must come out entirely including the tuber, because aphids will spread it through the whole patch. Powdery mildew starts on lower leaves and responds to a weekly spray of 1 tablespoon potassium bicarbonate per gallon of water on Tuesday or Wednesday when highs stay under 80°F.
- 3
On tomatoes, the real July watch is early blight and septoria leaf spot, not just late blight. Both start as small brown spots with concentric rings or yellow halos on lower leaves. Strip every leaf below the first fruit truss this week, mulch heavily with straw to block soil splash, and water at the base only. Morning irrigation before 8 a.m. gives foliage time to dry before the cool 55°F nights.
- 4
Stone fruit needs a shothole fungus check on Thursday when the low drops to 53°F. Rake every fallen cherry, plum, and peach leaf out from under the canopy and bag it. Small round holes with yellow halos are the tell. A dormant copper spray goes on the calendar for November, but right now sanitation is the only lever that works.
- 5
Cucumber and melon vines are entering bacterial wilt season. If a single leaf or vine wilts overnight while the rest of the plant looks fine, cut the stem and press the cut ends together; a sticky white string of sap when you pull them apart confirms bacterial wilt spread by cucumber beetles. Remove infected plants immediately and set out yellow sticky traps to knock down the beetle population before it hits the winter squash.
Frequently Asked Questions
Why are my raspberry canes dying back in the middle of summer in Portland?
Cane blight and spur blight are the two most common culprits in July, both fungal diseases that enter through pruning wounds or winter damage and show up now as the plant tries to ripen fruit. Cut affected canes to the ground, disinfect tools between cuts, and improve airflow by thinning the patch to 4 to 6 strong canes per running foot of row. Portland's humid mornings feed these fungi even during dry weeks.
How do I tell early blight from late blight on my Portland tomatoes?
Early blight shows as brown spots with concentric rings, almost like a target, and starts on the lowest leaves working upward slowly. Late blight moves fast with greasy dark patches on leaves and stems, often with white fuzz on the undersides after a humid night, and it kills plants in about a week. In July, early blight and septoria are far more common in Portland because rainfall is essentially zero, but any night above 55°F with dew still makes late blight possible on unsprayed plants.