✨ Spotted leaves, sticky aphids, and a Fourth of July heat spike — disease patrol in the dry weeks.
I spent Tuesday morning bagging up the worst of the black spot on Portland roses in my front bed, and the cherry tree out back is showing the tidy little holes that mean shothole fungus has settled in for the summer. With Saturday's forecast jumping to 85°F and zero rain in sight, this is the week to clean up fungal damage on roses and stone fruit while also watching beans for the first aphid colonies. I'll walk you through what I'm doing in three different corners of the garden so you can hit yours before the holiday weekend.
This Week's Action List
- 1
Black spot on roses: strip off every leaf with a black blotch or yellow halo, including any that have already dropped to the ground, and bag them (do not compost). I'm doing this Wednesday or Thursday morning before Saturday's heat, because handling roses in 85°F sun stresses the plant more than the disease does. Follow with a light feeding of alfalfa meal — a stressed rose fights fungus poorly.
- 2
Cherry and plum shothole fungus: rake every fallen leaf from under the tree and bag it. The fungus (Wilsonomyces carpophilus) overwinters in leaf litter, so sanitation now means fewer holes next May. Do not spray copper in July heat — wait until dormant season in late November for a copper application that actually does something.
- 3
Aphids on beans and nasturtiums: I'm scouting the undersides of bean leaves every other morning this week. A strong blast of water from the hose at 7 a.m. knocks 80% of them off and they rarely climb back. Skip the insecticidal soap when temps are above 80°F — it burns leaves in our dry July air. Save soap sprays for cooler mornings under 75°F.
- 4
Late blight check on tomatoes: even though July is drier, our 50 to 55°F overnight lows plus any morning dew can still trigger blight. Walk the tomato patch Friday morning before the heat spike and look for greasy gray to brown lesions on lower leaves. Snip and bag anything suspicious — do not shake the plant or you spread spores.
- 5
Powdery mildew on bee balm and phlox: this is the week it usually shows up on ornamentals, not just squash. Thin crowded stems to improve airflow (I pull out about every third stem on monarda) and water at the soil line only. Overhead sprinkler in July is an open invitation to mildew on every perennial you own.
Frequently Asked Questions
Should I spray fungicide on roses during a Portland heat wave?
No. Most fungicides, including copper and neem, can scorch rose foliage when temperatures climb above 80°F, and Saturday's forecast hits 85°F. I focus on sanitation (removing infected leaves) in hot dry weeks and save any spray applications for cool overcast mornings under 75°F, which we usually get back by late July.
Why does my cherry tree have small round holes in the leaves every July?
That's almost certainly shothole fungus, also called coryneum blight, and it's extremely common on Portland cherries, plums, and ornamental Prunus. The fungus infects leaves in our wet spring and the damaged tissue drops out as the leaf hardens off, leaving the BB sized holes. Rake and bag fallen leaves now, and plan a dormant copper spray in November after leaf drop.