The plum tree (Prunus domestica) is a fruit tree that is part of the Rosaceae family. It is valued for its fruit, which is used to make jams, preserves, and alcoholic drinks. The plum tree is not very demanding in terms of vegetation factors, making good use of hill soils. It does not tolerate drought and compact soils. The plum tree starts bearing fruit 4-5 years after planting and has a longevity of 30-40 years. The plum leaves are oval and have an evening edge, and the flowers are white. Plums can be exploited in different ways. They contain carbohydrates, organic acids, albumin, cellulose, vitamins (A, B, C) iron, calcium, magnesium, phosphorus, sodium.

The main diseases of plum trees

VIRUSES

Sharka (Plum pox virus)

Circular spots of discoloration appear on the leaves, covering all the foliage. The symptoms disappear in the hot summer months. Circular yellowish spots appear on the fruit, clearly visible on unripe fruit. Affected fruits fall from the tree before their time. In case of severe attack, circular spots also appear on the fruit pits. The virus is spread through infected plant material, aphids, and pollen.

Prevention and control measures:

• using healthy grafts and rootstocks;
• removing weeds and shrubs near orchards;
• insecticide treatments to control the insect population.

BACTERIOSIS

Bacterial canker of stone fruits (Pseudomonas syringae pv. morsprunorum)

This disease occurs on young leaves, twigs, and fruit. Small, circular, watery-looking spots appear on the leaves. When humidity is high, bacterial ooze appears on the spot surface and is a yellowish-white, viscous pellicle. The attacked tissues dry out and fall off the leaf in dry weather. Thus, the plum tree leaves have a pitted appearance. A strong attack causes tree defoliation. Small, pinkish-brown spots appear on the fruit. The attack causes the fruit to deform and the pulp to crack. This causes the plums to lose their commercial value. Long spots appear on the twigs, and the bark turns brown and gets damaged. The wounds produce a viscous ooze, specific to bacteriosis. The wounds increase in size from year to year and may evolve into open cancers. Bacteria enter the tree through stomata, but most infections occur during grafting.

Prevention and control measures:

• using healthy grafts and rootstocks;
• chemical treatments with copper-based fungicides.

Crown gall (Agrobacterium radiobacter pv. tumefaciens)

The disease appears as tumors (galls), which vary in size depending on the plant’s age and the affected organ. Initially, small, smooth, and soft tumors appear on roots, stems, and rarely on branches or leaves. With time, the tumors enlarge, become brown or blackish-brown, and acquire a rough appearance. The pathogen enters the plum tree through wounds caused by nematodes, hail, insects, etc. The appearance of this disease is also favored by temperatures between 22 and 30 °C and atmospheric humidity of 80 %.

Prevention and control measures:

• sorting plum saplings before planting;
• plum saplings should be planted on land that has been prepared accordingly;
• disinfecting scissors and tools when moving from one tree to another;
• pruning affected branches up to the healthy area, applying treatment with Bordeaux mixture, and covering wounds with tree wound sealer;
• soaking the sapling roots in a solution of Bordeaux mixture of 1%;
• treatments during vegetation with copper-based products.

MYCOSIS

Bladder plum gall (Taphrina pruni)

The young fruits take on a greenish-yellow color and become larger than the healthy ones. After the attack, the fruit is deformed and the seed husk is destroyed. A greyish velvety fuzz appears on the surface, representing the fungus fructifications. After 3-4 weeks, the fruits shrivel and fall off massively. The fungus overwinters as mycelium on the branches’ bark. The disease occurs in cold, wet springs.

Prevention and control measures:

• planting resistant plum tree varieties;
• chemical treatments with specific fungicides.

Shot-hole disease (Stigmina carpophila)

The disease is common in untreated orchards and causes significant damage to stone fruit trees. Circular spots appear on the leaves and the tissues turn brown and eventually detach from the rest of the leaf. Punctiform formations appear on the fruit, surrounded by a purplish-red border. The fruit pulp loses its texture and taste. The fungus causes browning of the fruit, followed by wounds. This form of attack is very dangerous because it dries out the fruit buds. The spores spread the fungus and resist over winter in the sap that drains from the wounds. The fungus can also overwinter as mycelium on the branch’s surface.

Prevention and control measures:

• pruning and burning the affected branches;
• covering the wounds resulting from prunings with tree wound sealer;
• after pruning, treating the plum trees with Bordeaux mixture;
• preventive treatments in vegetation with specific fungicides.

Powdery mildew (Podosphaera tridactyla)

The attack appears on young twig leaves as white spots. These spread and cover the entire leaf. The disease progresses and the mycelium becomes greyish and dusty. Attacked tissues shrivel and dry out. The fungus also attacks young fruit. These get covered with white mycelial fuzz. The fruit cracks and rots.

Prevention and control measures:

• balanced fertilization;
• planting resistant varieties;
• burning attacked twigs and fruits;
• chemical treatments in vegetation with specific fungicides.

Brown rot (Monilinia laxa)

This disease attacks all aerial organs of trees, causing significant damage in cold and rainy years. Affected leaves hang down without falling off, flowers brown and wither, and branches bend into a hook shape. Young fruit shrivels, browns, and falls massively. The pulp of mature fruit rots and yellowish-grey spots appear on the outside. Eventually, the fruits mummify and remain on the tree, ensuring disease transmission into the following year. The fungus overwinters on the mummified fruit and in the bark of attacked branches.

Prevention and control measures:

• gathering and destroying attacked fruits;
• pruning and burning of the affected branches;
• pruning the affected branches down to the healthy area, applying treatment with Bordeaux mixture, and covering the wounds with tree wound sealer;
• chemical treatments during the dormant period with copper-based products;
• preventive treatments applied in vegetation with specific fungicides.

Red spot disease (Polystigma rubrum)

Circular, yellow-cream-colored spots appear on plum leaves, which evolve to orange and finally red. The tissues around the spots thicken, become crusty, and slightly curved towards the lower part of the leaves. Eventually, the attacked leaves fall off, causing premature defoliation, thus the tree becomes susceptible to frost.

Prevention and control measures:

• gathering and burning leaves from the orchard;
• planting resistant plum tree varieties;
• carrying out treatments with specific fungicides.

Rust (Tranzschelia pruni-spinosae)

Yellow spots appear on the upper part of the leaf blade. Brown spores appear on the lower side, next to the spots, and later (towards the end of the vegetation) black resistance spores appear. The attack of this fungus causes low frost resistance and fruiting every 2 years. The fungus develops its first stages on various anemone plant species and then moves to fruit trees. Also, the blackthorn (Prunus spinosa) is an intermediate host where the fungus multiplies and then migrates to cultivated species. It overwinters as mycelium in the underground organs of anemone plant species.

Prevention and control measures:

• gathering and destroying affected leaves;
• removing Prunus spinosa and Anemone spp. species near orchards;
• chemical treatments with specific fungicides.

The main pests of plum trees

Codling moth (Cydia pomonella)

It is a polyphagous species that attacks several species of fruit trees. Larvae of different ages attack the fruit. The females lay their eggs directly on the fruit or on leaves near the fruit. After hatching, the larvae burrow into the attacked fruit, leaving droppings inside. This makes the fruit inedible.

Control methods:

• using pheromone traps to identify adult flight;
• chemical treatments with specific insecticides.

Brown-tail moth (Euproctis chrysorrhoea)

It is a polyphagous species that attacks several trees and fruit trees. It has one generation per year and overwinters like a larva in a cocoon at the top of the twigs. The larvae attack buds and leaves and may cause tree defoliation. To detect the presence of this pest, trees should be examined very carefully.

Control methods:

• removing branches on which the larvae are present;
• chemical treatments with specific insecticides.

Fall webworm moth (Hyphantria cunea)

It is a polyphagous species that attacks ornamental trees (mulberry, plane tree, etc.) as well as fruit tree species. It has two generations per year and overwinters as a pupa in the surface layer of the soil. This pest causes tree defoliation and thus lowers fruit production. The caterpillars settle on the underside of the leaves and begin to consume them. They also build a kind of nest, inside which they feed.

Control methods:

• chemical treatments with specific insecticides.

The peach twig borer (Anarsia lineatella)

This species is also found in plum trees. It develops three generations per year and overwinters as a larva under the exfoliated bark. In the spring, they leave the winter shelter and attack the buds. In young twigs, the larvae gnaw penetrating holes. Newly emerged females lay eggs on the buds, and the larvae will penetrate the fruits of the early varieties. The larvae of the next generations penetrate the fruits in the ripening phase, where they dig deep galleries.

Control methods:

• pruning and destroying the affected branches (twigs);
• chemical treatments with specific insecticides.

Spider mites

They are pests that are very difficult to see with the naked eye. Spider mites are polyphagous species that attack many plants and fruit trees, both cultivated and wild species. They feed on plant sap. After the attack, the leaves look parchment-like, the flowers abort and the tree stagnates in growth.

Control methods:

• performing treatments during the dormancy period ;
• chemical treatments with specific insecticides.

Oriental fruit moth (Grapholita molesta)

It has 3-4 generations per year and overwinters like a larva in a silky cocoon in the cracks of the bark. This moth attacks the twigs, leaves, and fruits of the plum tree, but it does the most damage to the fruit. The larvae of the first generations attack the twigs and feed on them, causing great damage to newly established orchards and nurseries. The larvae of generations III and IV cause significant damage to the fruit. They enter the fruit through the stalk area or the “tip” area and gnaw irregular galleries around the pit. Fruits stagnate from growth, rot, and fall.

Control methods:

• pruning and destroying attacked fruits and twigs;
• using attractant traps;
• chemical treatments applied during mass flight with specific insecticides.

San Jose scale (Quadraspidiotus perniciosus)

It is a polyphagous species that attacks over 200 species of plants. It has 1-3 generations per year and overwinters as larvae on affected species bark. Females and larvae spread to all organs of the affected plants, including the fruit, and feed on host plants sap. In case of a massive attack, the shields that protect the body of the insect overlap and suffocate the trees. They stagnate from growth, and after 2-3 years they dry out.

Control methods:

• cleaning the branches with a rough brush;
• applying dormant period treatments with horticultural oil-based products;
• chemical treatments with specific insecticides.

Aphids

They are polyphagous species that migrate from one plant to another or from one species to another. They grow on spontaneous flora and then move to cultivated species. Aphids form colonies on the underside of leaves, on flowers or inflorescences, and on young twigs. The insects sting and suck the plant’s sap, causing stress. In case of severe attack, the tree’s resistance to disease is reduced.

Control methods:

• chemical treatments with specific insecticides.

Plum fruit moth (Cydia funebrana)

This species attacks stone fruit trees. It develops two generations per year and overwinters in a cocoon under the bark of trees or in various sheltered places. The larvae penetrate the fruit and feed on the pulp and seeds. The attacked fruits stagnate from growth and fall. The larvae continue to feed on the fallen fruit. After feeding, they come out of the fruit and look for the right place for winter.

Control methods:

• chemical treatments with specific insecticides.

Sciaphobus squalidus

It is a dangerous species that attacks mainly buds and flower buds. It develops one generation every two years and overwinters as an adult in the topsoil. In spring, the adults emerge and climb the trees and feed on the buds and flower buds. The larvae are not harmful to trees, they feed on the roots of spontaneous herbaceous plants.

Control methods:

• soil mobilization in the orchard (autumn);
• chemical treatments with specific insecticides.

Apple blossom weevil (Anthonomus pomorum)

It develops one generation per year and overwinters as an adult in sheltered places (tree bark). Adults appear in spring and feed on young leaves. Females lay eggs in flowers or flower buds. The larvae feed inside the flowers or buds.

Control methods:

• chemical treatments with specific insecticides.

Thrips

They are small pests, difficult to see with the naked eye that attack a large number of crops or fruit trees (including plum trees). They colonize the growth tips, flowers, and leaves, causing flower abortion and tree growth stagnation.

Control methods:

• using attractant traps;
• chemical treatments with specific insecticides.

Apple leaf-curling midge (Dasineura spp.)

It develops 3-4 generations per year and overwinters in the topsoil. The first signs of the attack appear in May, as the leaves roll up and become discolored. Inside these leaves are worm-like, yellow-orange larvae. In case of a strong attack, photosynthesis capacity is reduced and so is the plum tree productivity.

Control methods:

• chemical treatments with specific insecticides.

The spotted tentiform leafminer (Phyllonorycter blancardella)

This pest attacks a range of ornamental shrubs and fruit trees (including the plum tree). The larvae feed on the tissues between the upper and lower leaf epidermis. A massive attack can severely reduce photosynthesis capacity and thus lower yields.

Control methods:

• collecting and destroying fallen leaves on the ground;
• chemical treatments with specific insecticides.

Plum scale (Sphaerolecanium prunastri)

It is a polyphagous species that attacks several fruit trees (including the plum tree), vines, etc. It develops one generation per year and overwinters as a larva on the bark of attacked branches. Adults and larvae colonize the branches causing tissue necrosis. They can also attack the leaves, leading to their deformation and fall. The attacked plants are covered with sweet droppings, which favors the appearance of phytopathogenic fungi.

Control methods:

• cleaning the branches with a rough brush;
• applying dormant period treatments with horticultural oil-based products;
• chemical treatments with specific insecticides.

Winter moth (Operophtera brumata)

It is a species that develops one generation per year and attacks several ornamental trees and fruit trees. On plum, the larvae cause significant damage. They attack the buds, flowers, leaves, or even the fruit. After the attack, fruit buds are destroyed, floral organs are eaten and leaves are gnawed. As a result, production drops significantly.

Control methods:

• chemical treatments with specific insecticides.

Sawflies (Hoplocampa minuta, Hoplocampa flava, Eurytoma schreineri)

The larvae of these wasps enter young fruit, where they consume the pit and some of the pulp. After the attack, the fruit shrivels, turns purple, and falls from the tree.

Control methods:

• chemical treatments with specific insecticides.