Plum

In February or March, while the tree is dormant and temperatures are below freezing, find the hard black swollen galls on twigs and branches and cut at least 4 inches below each one, back into clean wood. Carry the cut wood out of the orchard and burn, bury, or bag it - do not leave prunings on the ground. The fungus Apiosporina morbosa overwinters alive inside the gall and fires spores at young shoots during spring rains, so a gall left on the ground or in a brush pile keeps infecting the tree it came from. Cutting well below the knot matters because the fungus grows inside the branch further than the visible swelling.

While the tree is bare and still freezing, find the black, cracked, charcoal-looking swellings on twigs and limbs and cut at least 4 inches below each one, back into clean wood. Carry the prunings off the site and burn or bury them, and cut the knots out of any wild plum or chokecherry thicket within throwing distance. The knots are the fungus itself, and they pump out spores every spring rain until they are gone; prunings left lying under the tree keep releasing spores as if they were still attached, and wild Prunus in the fencerow reinfects the tree you just cleaned.

Prune after the coldest weather has passed but before growth begins. Train to an open center, vase-shaped, cutting close to the branch collar and taking out crossing and inward-growing wood so light and wind reach the middle of the tree. Wild plum and its hybrids throw long, upright branches with few laterals, and a closed canopy stays wet after rain, which is exactly what brown rot and black knot need to infect.

In spring, measure last season's new shoot growth before reaching for fertilizer. A young plum should put on 10 to 20 inches of new shoot each year and an established tree 8 to 10 inches. If it is falling short, spread a half cup to a cup of 20-0-0 around the dripline of a young tree, or 1 to 2 cups for a tree five years or older. If growth is on target or excessive, apply nothing. Nitrogen is nearly always the only nutrient a plum lacks, and shoot extension is the tree's own honest report on whether it has enough. Overfeeding buys soft vigorous wood that is more attractive to aphids and slower to harden off for winter, and it costs you fruit.

Remove dead damaged and crossing wood after the coldest weather passes; keep the center open for sun and airflow. Open growth reduces brown rot pressure and helps ripen fruit evenly.

On plum, spray horticultural oil at the green cluster stage, after buds break but before bloom, at a 2 percent rate, using the dilution your product's label gives for it. Cover the bark and crotches thoroughly; the oil only works where it lands. Oil kills San Jose scale, aphid eggs, and mite eggs by smothering them while they are exposed on the bark and before leaves shelter them. Once the canopy fills in, coverage of overwintering stages becomes impossible.

Begin fungicide applications when the flower buds are just starting to open, using captan, chlorothalonil, thiophanate-methyl, or lime sulfur, and repeat per the label until the new shoots have matured and hardened off. Spores are shed from last year's knots during spring rains and infect young green shoots and wounded branches, which is why the spray has to be running before the new growth is out and why fresh pruning cuts made in wet spring weather are an open door. Once shoot growth hardens, susceptibility drops and spraying stops paying.

On a tree with a history of knots, begin fungicide when the flower buds are just beginning to open, using captan, chlorothalonil, thiophanate-methyl or lime sulfur, and continue on the label interval until the new shoots harden off and the weather turns consistently warm and dry. Black knot spores are shot out of old knots during spring rains and can only infect the current season's green shoots, so the whole spray program is about protecting that soft new wood while it is still soft.

Start a fungicide when the first blossoms open, using myclobutanil, propiconazole, chlorothalonil, captan, or sulfur, and repeat per label through petal fall. Monilinia spores infect through the open flower, and the blighted blossom becomes the base camp from which the fungus moves into the spur and twig and produces the spores that rot the fruit months later. Stopping it at bloom is what makes the summer manageable.

If the tree has rotted fruit before, spray a fungicide labeled for plum, captan, myclobutanil or propiconazole, when about a quarter of the flowers are open, then again 10 days after shuck fall and once more 10 days after that. Monilinia infects open flowers, and a blossom needs only about three hours of wetness near 70F to take the fungus in; the blighted flower then becomes a twig canker that seeds the fruit rot in August.

On stone fruit, time the plum curculio spray for shuck split, after the petals have dropped and the dried flower remnants are splitting off the young fruit. Kaolin clay (Surround) is an option for home plantings. Repeat if warm weather continues, tightening the interval during peak activity, and pay extra attention to trees along a woods edge. The overwintered adults walk out of the leaf litter and hedgerows and cut crescent-shaped egg scars into the young fruit; the grub then tunnels the fruit centre and the fruit drops. The first few days above roughly 75F after petal fall are when the beetles are actually active and hittable - below 70F they barely move.

Once three quarters or more of the petals have fallen and the fruit is around a quarter to a half inch, spray malathion or permethrin, or use kaolin clay for a non-insecticide option, and repeat on a 10 to 14 day interval. Trees next to woods need the follow-up most. The weevil overwinters in leaf litter at the woods edge, walks into the tree during bloom, and cuts a crescent-shaped slit in each young fruit to lay an egg - the first warm spell after petal fall, above about 70 to 75F, is when nearly all of that egg-laying happens. Mississippi State calls malathion the most effective material available to a homeowner for this weevil.

When fruitlets size up, hand-thin clusters so remaining plums have room to develop without limb breakage. Thinning improves fruit size and reduces broken branches on heavy-setting plum trees.

Once the danger of frost has passed, look at the crop load before you thin anything. American plum and most of its hybrids do not generally need thinning unless you want larger fruit - Alderman in particular carries a full load without help. If the tree is an overbearer like Waneta, or you want bigger plums, break up the clusters and thin to a single plum every 4 to 6 inches of branch, keeping the fruit nearer the trunk end of the limb. Utah State scopes the 4-to-6-inch spacing to European and Japanese plums and says American plums and American hybrids generally do not require it. But some of the hybrids do overset badly: the Wisconsin Carandale trial found Waneta over-fruits to the point that fruit size and quality drop off sharply, and a heavy load on the long branches of these trees breaks wood - while touching fruit in a cluster hands brown rot a bridge from one plum to the next.

Through the growing season, cut the shoots coming up from the roots off flush with the ground if you are training a single-trunk tree. If you want the thicket for wildlife cover and a hedge, leave them and let the colony fill in. Prunus americana suckers freely and will build a dense colony from its roots - that is how it lives in the wild, and it is a choice, not a defect, but a thicket is far harder to prune knots and curculio out of than a tree.

During the early fruit drop after petal fall, walk the tree at least twice a week and promptly collect every fallen plum, looking for the crescent-shaped scar. Destroy them - do not compost them under the tree. The larva rides the fruit to the ground and spends only about two to three weeks feeding inside it before exiting to pupate in the soil and emerge as the next generation of adults; picking the drops up promptly, before the larva leaves the fruit, breaks that generation instead of raising it.

Start fruit-rot sprays 2 to 3 weeks before harvest as the fruit begins to soften and colour, using captan, myclobutanil, propiconazole, or sulfur, and repeat per label right up to harvest. Green fruit is largely immune; susceptibility climbs steeply as sugars rise during ripening, which is why a clean tree can go to mush in the last fortnight. Warm wet weather in that window is what turns a few infections into a total loss.

Begin fungicide about one month before the plums start to color and repeat every 7 to 10 days through harvest, using a product labeled for plum such as captan. The second brown rot infection window opens as sugar rises in the ripening fruit - one rotted plum sporulates and takes the whole cluster down within days of picking weather.

Pick up fallen fruit through the summer and after harvest, and before bud break next spring strip every shrivelled mummy still hanging in the tree. Cut out twigs killed by brown rot and any cankered branches in February or March. Bury the infected fruit in a compost pile rather than tossing it under the tree. Mummies and cankers are where Monilinia overwinters - each one is a spore cannon aimed at next year's blossoms. Dropped, grub-infested fruit is also how plum curculio larvae reach the soil to pupate, so clearing drops cuts next season's beetle count.

After harvest, pull every shriveled, hard, gray-fuzzed plum off the tree and rake up the ones on the ground, and cut out the twigs with sunken, gummy cankers. Finish before bud break next spring. The fungus overwinters in those mummies and in the infected wood, and they are the entire source of next spring's blossom blight - a mummy left hanging is next year's inoculum sitting directly above the flowers.