Flowering Crabapple

In late winter before growth resumes, take out dead, crossing, and rubbing branches, thin the interior to open the canopy, and cut off the suckers rising from the base and the water sprouts standing straight up in the limbs. Aim for a canopy that lets air move through it. Pruning now reduces sucker regrowth from wounds and eliminates the chance of spreading infection between cuts, because the fire blight bacterium is dormant and not moving in the sap. An open canopy also dries the leaves quickly after rain, and apple scab and fire blight both need wet tissue to infect.

Send a soil sample to your extension lab and fertilize only to the rate it calls for. An established crabapple in decent soil often needs nothing. Resist the urge to push a slow tree with a heavy nitrogen feed. Excess nitrogen stimulates a flush of succulent leaves and shoots, and that soft new growth is exactly the tissue fire blight infects most easily. A modestly fed crabapple is a more disease-resistant crabapple.

When the buds crack and the first green leaf tissue shows, spray horticultural oil at the 2 percent delayed-dormant rate — your product's label gives the dilution that reaches it — covering twigs and branches to the point of drip. Green tip through half-inch green is the window on apple and crabapple. Overwintering aphid eggs, San Jose scale nymphs, and mites are resuming metabolic activity as temperatures moderate, so they are breathing hard enough for the oil film to suffocate them. Earlier, they are too dormant for it to work; later, the leaf tissue is too tender to take the oil.

If your tree is a scab-susceptible cultivar and defoliated last summer, start captan once leaves reach a half to three-quarters of an inch, and continue through pre-pink and pink. On a resistant cultivar, skip this entirely. Venturia inaequalis overwinters in last year's fallen leaves and fires its spores at the youngest expanding leaves during spring rains. The fungicide is protective, not curative, so it must be on the leaf before the rain lands, and once the earliest lesions form they seed the rest of the season.

As the petals drop, continue on a susceptible tree by rotating captan, sulfur, and myclobutanil through the cover-spray period. Myclobutanil covers cedar-apple rust and powdery mildew as well as scab, so one rotation handles all three. Petal fall is when the bees leave and the young leaves and fruitlets are still expanding and still infectable. Cedar-apple rust needs eastern red cedar as its alternate host and moves cedar to apple in mid to late spring, so rust pressure is real only if junipers are nearby.

Watch for shoots whose tips wilt, blacken, and curl into a shepherd's crook. Cut through healthy wood at least 8 inches below the discolored bark of the canker, and further, 12 to 18 inches, on an actively running strike. Remove the prunings from the orchard. Erwinia amylovora enters through blossoms during warm wet spring weather and then runs down inside the shoot ahead of any symptom you can see, so a cut close to the visible damage leaves live bacteria in the tree.

Harvest when the small apples have taken full color and come away easily. Large-fruited jelly types like Dolgo ripen in midsummer and are tart enough to eat raw; most ornamental crabapples are best cooked into jelly or cider. Leave what you do not want on the tree, since persistent-fruited cultivars hold their fruit into winter and feed birds. Crabapple fruit is edible but too tart and firm to be palatable raw on most ornamental cultivars, so it is cooked. Fruit left on a persistent-fruited cultivar hangs into winter and feeds birds, which is the reason to pick only what you want.

After leaf drop, rake up and remove the fallen leaves from under the canopy, or shred them and bury them under fresh mulch so they rot down before spring. The scab fungus overwinters in that leaf litter and nowhere else, and the spores it releases next spring are the entire source of early infection. Removing the leaves removes the inoculum before it can be launched.