Pests
Symptoms On small developing plums, look for a curved, crescent-shaped scar, the mark the beetle leaves when it lays an egg. Damaged fruit often drops early, and some that stays on the tree ends up misshapen or has a grub inside. You see this soon after the petals fall, and it is the usual reason for wormy plums.Small developing fruit carries a distinctive crescent-shaped scar where the female cut a slit to lay an egg. Infested fruit often drops early; fruit that hangs on may be gnarled or have a grub tunneling inside. Damage appears shortly after petal fall and is one of the most common causes of wormy backyard plums. Cause A small beetle with a long snout called the plum curculio, which is named for this very crop. The adults show up around bloom and start damaging the little fruits right after the petals drop, laying eggs inside them. The larvae then feed within the fruit and later drop to the ground.The plum curculio (Conotrachelus nenuphar), a small snout beetle and a signature pest of its namesake crop. Adults emerge around bloom and attack fruit from petal fall onward, laying eggs in the developing fruit; the larvae feed inside, then drop to pupate in the soil. Organic treatment Early in the morning the beetles are slow. Spread a sheet under the tree, tap the branches so the beetles fall, and destroy them. Pick up and throw away any dropped fruit promptly so the larvae inside do not mature. A kaolin clay spray (a fine white mineral coating) starting at petal fall helps keep them off.For a backyard tree, jarring the limbs over a sheet in the cool early morning when the beetles are sluggish, then destroying what drops, reduces the population. Promptly gather and dispose of dropped infested fruit to break the cycle. Kaolin clay applied from petal fall is an organic deterrent. Prevention The main thing is cleanup. Pick up fallen fruit all season so the larvae inside cannot finish growing in the soil under the tree. Keeping the ground clear lowers the number of beetles the next year.Sanitation is the backbone: remove dropped fruit through the season so larvae cannot complete development in the soil beneath the tree. A clean orchard floor and prompt fruit pickup steadily lower pressure year over year.
Symptoms In spring the new leaves at the shoot tips curl up tightly, and if you unroll one you find little green or waxy-gray insects, aphids, packed inside. They leave a sticky shine (called honeydew) that turns black with sooty mold. A bad early attack curls the leaves and slows the new growth.In spring, new leaves at the shoot tips pucker and curl tightly around colonies of small soft-bodied aphids (leaf curl plum aphid), while the pale, wax-dusted mealy plum aphid clusters on the undersides of leaves. Both excrete honeydew that turns leaves shiny and sticky and grows black sooty mold; heavy early infestations stunt shoots and can distort young fruit. Cause Two kinds of aphids that live on plum: one curls the new leaves, and the other is a pale, waxy gray one. They spend the winter as eggs near the buds, then multiply fast on the tender spring growth before flying off to other summer plants.Two aphids specific to plum: the leaf curl plum aphid (Brachycaudus helichrysi), which curls the spring foliage, and the mealy plum aphid (Hyalopterus pruni), whitish with a mealy wax coat. Both overwinter as eggs near the buds, build up on new spring growth, then migrate to summer hosts (leaf curl plum aphid to plants in the aster family, mealy plum aphid to reeds and cattails). Organic treatment Ladybugs and other helpful bugs usually clean aphids up by early summer, so avoid harsh sprays that kill those helpers too. Blast the colonies off with a hard spray of water, and for a bad case use insecticidal soap or a light horticultural oil on the curled shoots. A dormant oil spray in late winter kills the overwintering eggs.Natural enemies (ladybugs, lacewings, syrphid larvae, parasitic wasps) usually bring aphids under control by early summer, so protect them by not spraying broad insecticides. A strong jet of water knocks colonies off, and narrow-range horticultural oil or insecticidal soap on the affected shoots helps in a heavy infestation. A dormant or delayed-dormant oil spray targets the overwintering eggs. Prevention Go easy on spring feeding, since lush new growth attracts aphids. Keep ants off the trunk (they protect aphids) with a sticky band, and leave the helpful bugs alone. If aphids come back every year, a dormant oil spray in late fall or winter is the best prevention.Avoid heavy spring nitrogen, which pushes the soft flush aphids favor. Keep ants (which farm aphids for honeydew and drive off predators) off the trunk with a sticky band, and preserve natural enemies. A late-fall or dormant oil where aphids are a chronic problem is the most effective single measure.
Symptoms Look at the very bottom of the trunk where it meets the soil. The peachtree borer, which also attacks plums, leaves a sticky, gummy ooze mixed with what looks like wet sawdust (frass). You may see a small mound of gum and frass at the base. A young tree with several borers can be killed in one season, so catch this early.Gummy ooze mixed with sawdust-like frass at the base of the trunk, right around the soil line, often with a mounded look where the bark meets the ground. Plums, like other stone fruit, are a host. A heavily infested young tree loses vigor as the larvae girdle the inner bark; several borers in one trunk can kill a newly planted tree in a season. Cause The larvae (young grub stage) of the peachtree borer, a moth whose adults look like wasps and fly by day. The adults lay eggs on the lower trunk in summer, and the larvae burrow into the bark at and below the soil line. Trees with existing wounds are hit hardest.Larvae of the peachtree borer (Synanthedon exitiosa), a clearwing moth whose day-flying adults are wasp-like. Eggs are laid on the lower trunk in summer; larvae tunnel in the inner bark at and just below the soil line. Trees stressed by winter injury, mechanical wounds, or bacterial canker are attacked first. Organic treatment Once the larvae are tunneling inside, no spray reaches them. The backyard fix is to scrape soil away from the base in late summer or fall, find the holes marked by gum and frass, and kill the larvae with a thin, stiff wire pushed into each hole. Then smooth the soil back.There is no rescue spray once larvae are tunneling inside. The practical home control is to expose the crown in late summer or fall, find the entry holes by the gum and frass, and kill larvae by hand with a thin wire. Keep the trunk base free of wounds. Prevention Keep mowers and string trimmers away from the trunk; the wounds they leave attract the egg-laying moths. Keep grass and weeds cleared from the base so you can see the soil line, and check it every late summer for fresh gummy ooze.Protect the lower trunk from mechanical injury (the wounds that draw egg-laying females), keep the tree vigorous, and clear weeds and tall grass from the base so the crown stays open and easy to inspect. Check the soil line each late summer for fresh gum.
Symptoms Little gray, round, hard bumps crust over the bark of twigs and branches, and the fruit can get small reddish spots where the scales feed. A bad infestation makes the bark look ashy and can kill off twigs and branches, weakening the tree.Tiny gray, circular, hard-shelled scales encrust the bark of twigs and branches, sometimes so densely the bark looks ashy; feeding sites on fruit show as small red-purple haloed spots. Heavy infestations kill twigs and whole limbs from the top down and weaken the tree over a few seasons. Cause An armored scale insect called San Jose scale. After the tiny young 'crawlers' settle down, they stay put under a hard shell and suck sap, with several generations a year. Only the crawler stage moves around and can be targeted.San Jose scale (Diaspidiotus perniciosus), an armored scale insect. Immobile once settled, it feeds on sap under a waxy shell and passes several overlapping generations a year; crawlers (the mobile young stage) are the only vulnerable window and spread the infestation along the branches. Organic treatment The main control is a thorough spray of horticultural oil in late winter while the tree is leafless, coating all the bark to smother the scales. During the season, helpful insects keep light infestations down. Prune out and destroy branches that are heavily crusted with scale.A thorough dormant or delayed-dormant application of horticultural oil, wetting all the bark, smothers overwintering scales and is the mainstay of home control. In-season, natural enemies (tiny parasitic wasps, predatory beetles) suppress light infestations; prune out and destroy heavily encrusted wood. Prevention Prune for good airflow, use a dormant oil spray in areas where scale has shown up before, and check any new trees you buy, since scale often hitches in on nursery stock. Avoid harsh sprays that kill the helpful insects.Keep the tree vigorous and well pruned so airflow is good, apply dormant oil where scale has appeared before, and inspect incoming nursery stock, since scale often arrives on new trees. Preserve natural enemies by avoiding broad-spectrum sprays.
Diseases
Symptoms Hard, black, warty lumps (called knots) grow along the twigs and branches, anywhere from an inch to six inches or more long. They start as a soft olive-green swelling one year and turn coal-black and hard the next. The branch beyond a knot often wilts and dies, and a badly infected tree gets stunted and gnarled. Black knot is the classic plum disease, the one to learn to recognize.Hard, black, elongated, warty swellings (knots) grow along twigs and branches, from an inch to six inches or more long, girdling and eventually killing the wood beyond them. A new gall starts as an inconspicuous olive-green, corky swelling one year, then hardens and turns coal-black by the second. Leaves on knotted branches may wilt and die, and a heavily infected tree becomes stunted and misshapen. This is the signature disease of plum. Cause A fungus (Apiosporina morbosa) that lives in the black knots over winter, on your tree and on any wild plums or cherries nearby. In cool, wet spring weather it shoots out spores that infect the new green shoots. You will not see anything the first year; the knot swells the next year and turns black the year after.The fungus Apiosporina morbosa (formerly Dibotryon morbosum). It overwinters in the galls on the tree and on nearby wild or ornamental Prunus, and in cool, wet spring weather releases spores that infect the new green shoots. Infection is invisible the first season; the gall swells the following year and blackens the year after, so the disease runs on a roughly two-year cycle. Organic treatment Cutting the knots out is the main fix, and when you do it matters: prune in late winter while the tree is bare, before it starts growing in spring. Cut each knot off 3 to 4 inches below the black lump, into clean wood, because the fungus reaches farther than you can see. Carry the cut pieces away and burn, bury, or bag them so they cannot re-infect the tree, and wipe your pruners with rubbing alcohol between cuts.Pruning is the core control, and timing is everything: in the dormant season, before growth resumes (before about April 1), cut out every knot, making the cut 3 to 4 inches below the visible gall into clean wood, since the fungus extends beyond what you can see. Remove the prunings from the area and burn, bury, or bag them, and disinfect tools between cuts. On a scaffold too valuable to remove, cut the gall out to healthy wood. A protective fungicide on the new shoots in early spring supports the pruning where pressure is high. Prevention Check the tree every winter and cut out any knots before spring, when they spread. Just as important, clean up or remove wild plums, chokecherries, and neglected ornamental plums nearby, because they keep re-infecting your tree. Some plum varieties resist black knot better than others, so ask when you buy, but cutting the knots out each year is the habit that matters most.Scout and prune out knots every dormant season before they can release spores, and deal with the reservoir: remove or clean up wild plum, chokecherry, and neglected ornamental Prunus within range, which sustain the disease. Choosing a less-susceptible cultivar and keeping the tree open and vigorous both help, but annual knot removal is the non-negotiable habit.
Symptoms At bloom, infected flowers go limp and brown, and small gummy dead patches can form on the nearby twigs. As the plums ripen, brown spots appear and spread quickly, sometimes rotting a whole fruit in a day or two in warm, wet weather, with a dusty tan fuzz on the surface. Rotted fruit shrivels into a hard, dried mummy that hangs on the tree.At bloom, infected blossoms wilt and turn brown, and the infection can run back into the twig to form a small gummy canker. As fruit ripens, brown circular spots appear and spread fast, sometimes rotting a whole plum within a day or two in warm, wet weather, with a dusty tan spore mass on the surface. Rotted fruit dries into a mummy that clings to the tree or drops. Cause A fungus called Monilinia fructicola. It survives the winter in dried mummy fruit and in dead twig patches, then releases spores at bloom that infect the flowers. Those flower infections later cause the fruit to rot. Warm, humid, wet weather makes it spread fast, and any break in the skin lets it in.The fungus Monilinia fructicola. It overwinters in mummified fruit (whether hanging in the canopy or fallen) and in twig cankers; spores released at bloom infect flowers, and those infections later seed the fruit rot. Warm, humid, wet conditions drive rapid spread, and any skin break (curculio scar, hail, cracking) is an entry point. Organic treatment The main control is cleanup: take off and destroy every dried mummy from the tree and the ground, and prune out the gummy dead twig patches, since these hold the spores. If the weather is wet during bloom, an organic-approved spray at bloom helps protect the flowers. As soon as you see a plum rotting, pick it and throw it away, and thin the fruit so plums are not touching.Sanitation is the core control: remove and destroy every mummy from the tree and the ground, and prune out gummy twig cankers, because these are the spore sources. During a wet bloom, an organic-approved bloom-time fungicide reduces blossom blight. Once fruit is rotting, pick and dispose of it promptly, and thin fruit so plums do not touch, which slows spread through a cluster. Prevention Cleanup all year prevents most of the damage: never leave dried mummies on the tree over winter, pick up dropped fruit, and prune so the tree is open and the plums dry fast after rain. Keep pests like plum curculio in check, since their wounds let rot in. Pick fruit on time, handle it gently, and chill it soon after picking.Year-round sanitation prevents most loss: never leave mummies on the tree over winter, remove dropped and thinned fruit, and keep the canopy open with good pruning so fruit dries quickly after rain. Control plum curculio and other pests whose wounds open the door, harvest promptly, handle fruit gently, and cool it soon after picking.
Symptoms Leaves get small angular water-soaked spots that turn dark and fall out, leaving little holes, called a shot-hole look. Bad cases turn leaves yellow and thin out the tree. On the fruit, small sunken spots, pits, and cracks form. How badly a tree is hit depends a lot on the variety, and Japanese plums are often more affected than European ones.Small angular, water-soaked spots on leaves turn brown to purple-black and drop out, leaving a shot-hole look; heavy infection yellows and thins the canopy. On fruit, small sunken spots, pits, and cracks develop on the skin, downgrading the crop. Susceptibility varies sharply by cultivar, and Japanese plums are often hit harder than European. Cause A bacterium (a microscopic organism, not a fungus) called Xanthomonas. It spreads in warm, wet, windy weather and builds up when there are other susceptible plum or peach trees close by. The spots can look like spray damage.The bacterium Xanthomonas arboricola pv. pruni. It spreads in warm, wet, windy weather and builds up where susceptible plum, peach, and related trees grow nearby. Symptoms are sometimes mistaken for spray injury or nutrient problems. Organic treatment There is no good home cure once it shows up. Copper sprays help a little as prevention but can hurt the tree if overused, so they are not a dependable fix. The best you can do is keep the tree healthy and the canopy open so it shrugs off the damage.Home options are limited once symptoms appear: copper sprays help more as prevention than cure and can injure the tree if overused, so they are not a reliable home fix. Keeping the tree vigorous and the canopy open and dry helps it tolerate the disease. Prevention The best prevention is the variety you pick. Choosing a plum known to resist or tolerate bacterial spot avoids most of the trouble, so it is a decision to make before planting. Good airflow, going easy on nitrogen, and a healthy tree also help.The most effective measure is choosing a resistant or tolerant cultivar at planting; this is largely a variety decision made before the tree goes in. Good airflow, avoiding excess nitrogen, and tree vigor reduce severity.
