What is apple tree rot and what are its types?

symptom

Rot damages bark, mainly in trunk, big branches, lateral branches, but also small branches, and occasionally the main roots are damaged. Symptoms generally show two types:

1. Ulcer type

In summer, the disease began to occur, mainly in the trunk, main branch, bifurcation, branchlet base, shear saw mouth and the healing part of shear saw mouth. At the beginning of the disease, the affected part is slightly wet, reddish brown, with uneven outline, the size of nails, and the length and width vary from tens of centimeters. When the diseased skin is sliced, the diseased tissue is soft, rotten, reddish brown, 2-3 mm deep, with deep spots and spots, and even the rotten skin reaches the xylem. In the decaying part of bark, the superficial lesions extend widely, while the deep lesions are smaller. Later, the expansion stopped and was slightly depressed. Black spots protruding from the bark surface are fruiting bodies produced by bacteria. Those that do not produce fruiting bodies are easily confused with the dry bark on the bark surface. During the growth period of fruit trees in summer and autumn, the disease spots are generally confined to the surface layer of bark, or spread along the surface layer of bark, and only spread locally to the deep. This type of symptom is called surface ulcer.

At the end of autumn and the beginning of winter, the hyphae in the diseased tissues gather to form a white hyphae mass, which breaks through the periderm and invades the internal healthy tissues. The diseased spots develop deeper into the bark, and reddish-brown to brown necrosis spots appear at the junction between the diseased tissues and the healthy tissues, which further expands the fusion and leads to bark rot. At the same time, necrotic spots with the same characteristics appear at the junction of healthy bark and dry bark on the bark surface, which continue to spread to the deep layer after winter.

In early spring, the lesion spread rapidly, and the affected part was reddish brown, waterlogged and slightly swollen. The outline was oval or rectangular, with different sizes, with a length of more than 1m and a width of more than half. The texture of the diseased tissue is not good, giving off the smell of distiller's grains, and some overflow reddish-brown juice. In spring, it is generally found from the appearance that the bark has rotted at the time of onset, and the lesion range of the inner layer of the bark is far greater than that seen from the appearance.

As the apple tree opens its leaves and blooms into the growing period, the activities of the diseased parts gradually stop, the water loses and sinks, and the color changes from dark brown to black. The affected area bulges around and forms an ulcer. Conidia were produced in diseased tissues, which broke through the epidermis and black spots appeared on the bark surface. When it rains or the weather is wet, golden yellow filamentous curly spore horns spew out. Some small spots in spring stop moving to form dry spots, which are buried under rough skin and continue to spread after winter. This situation is more common in the central region.

2. Branch withering type

In spring, the branchlets come on, and the diseased spots spread rapidly, without waterlogging, and the edges are not obvious, which spread quickly to the whole branches, causing the branches to wither. Extremely weak or suffering from freezing injury, sometimes showing symptoms of branch blight.

The diseased branches and branches are not flourishing, because the transport of nutrients is blocked, and there are many abnormal phenomena above the diseased branches. When the diseased part extends around the branch or trunk for a week, the diseased branch or even the whole plant will dry up and die.

Pathogen (Figure 1- 1)

Fig. 1- 1 apple tree rot fungus 1. Spore 2. Conidia and conidiophore 3. Ascomycetes 4. Ascomycetes and Ascomycetes.

The pathogen of apple tree rot is apple rot. )maire。 Mycelia spreads in bark disease tissues and can also enter xylem under diseased skin. The hyphae separated and became colorless at first, then turned olive to dark green. The outer matrix is formed under the epidermis, conical, and contains a conidia cavity, which is first single-chambered, then multi-chambered, with a hole. The diameter of conidia is 5 17- 1560μ m, the height is 430- 1300μ m, and the inner wall conidiophore is dense. Conidiophore has no branches or branches, and is 12-2 1μ m in length and 1.2-2.0μ m in width, with conidia at the top. Conidia are unicellular, colorless, curved, sausage-shaped, with round ends, 3.6-6.0×1.2 microns ... The conidia are embedded in colloid, and are ejected when the weather is wet, forming spore angles. Spore horn dissolves in water.

Endosteum is formed under the outer matrix and mixed with host cells. There is a clear black boundary between the internal or external matrix and the host tissue. The inner base contains 3- 14 cysts. Ascomycetes are spherical or nearly spherical, with a diameter of 34 1-495μ m, a neck length of 264-550 μ m, and densely distributed at the base. Ascomycetes are oval or spindle-shaped, with round or flat top, nearly sessile, 2 1-30× 6 microns, colorless walls and slightly thick top. Ascomycetes contain 8 ascospores, which are diallel or irregular. Ascomycetes are unicellular, colorless, similar in shape to conidia, but larger than conidia, with a size of 6.0—8.4× 1.8 μ m. ..

The optimum growth temperature of the pathogen is 28-32℃, the highest is 37-38℃, and the lowest is 5- 10℃. The optimum carbon source for mycelium growth is maltose, and the optimum nitrogen source is asparagine and nitrate. Comparing the effects of different auxins on mycelium growth, thiamine is the best.

Conidia and ascospores germinate well in apple juice, bark juice, dried apricot juice, maltose solution and sucrose solution. Conidia cannot germinate in distilled water. Although ascospores can germinate, it takes a long time and the germination rate is low. The optimum temperature for spore germination is 24-28℃. After 32 hours at 65438 00℃, the germination rate of conidia and ascospores can reach 30-40%. Indoor, the dispersed conidia lose their germination ability after 2-4 days, and the ascospores can survive for 13-20 days. After two days in the sun, the two spores lost their vitality.

Occurrence law

Apple tree rot is an infectious disease caused by weak parasitic fungi. Rotting bacteria can grow and reproduce on necrotic bark tissue. Bacterial propagules produced on diseased skin, dry hemorrhoids and dead skin constitute the source of infection of this disease. Conidia began to form after the onset of spring/0/6-35 days, and conidia were produced. Ascomycetes began to form in autumn, and a large number of ascospores were produced in the following spring. Spores of pathogenic bacteria are mainly transmitted by wind and rain. There are germ spores flying in the air of orchard all the year round, which increase after rain.

Rot bacteria invade through wounds, fruit stalk marks, leaf marks and lenticels. In Xingcheng, Liaoning Province, infection can occur from March to 165438+ 10, and the infection is most concentrated from late March to mid-May. After the invasion, when the host tissue is healthy and vigorous, the bark carries germs without getting sick. Only when the infected tissue is dead or dying, and the host's resistance is weak, the bacteria can expand, leading to bark disease. The pathogen has the characteristics of latent infection. Apple barks that are disease-free in the field are generally infected with bacteria, and the rate of bacteria in branches is much higher than the incidence rate, and the distribution area of bacteria is larger than the epidemic area of diseases. Whether the latent pathogen can spread and cause disease depends on the host's ability to resist transmission.

The spread of pathogenic bacteria requires a certain amount of dead tissue around the infection site, and it must grow on the dead tissue first to accumulate infection potential. In the field, the pathogens invading trees are easy to spread in frostbite and sawing, and mainly spread in the deciduous layer of bark, and then invade the healthy tissue of bark. Since summer, bark has formed a deciduous layer, and diseases have begun to occur. After the peak of winter and spring, apple trees enter the growth period and stop in the following spring, which is a process of decay.

According to the observation in Fengxian county, Shaanxi province, in summer, from early June to August, apple bark forms a peeling layer. The formation of litter layer provides a suitable basis for the activities of pathogenic bacteria. From mid-July to September, pathogenic bacteria spread in dead but not dried deciduous tissues, causing bark lesions. During the growth period of fruit trees in summer and autumn, the host has strong vitality and strong anti-expansion ability. The disease spots are generally confined to the surface layer of bark, or only partially spread deep, and the bottom layer is limited by periderm, forming surface ulcers.

From the end of autumn to the beginning of winter, from the end of 10 to the beginning of 1 10, as the fruit trees gradually entered the dormancy period, the resistance weakened and the disease spots gradually developed deep into the bark. The hyphae in the surface ulcer tissue gather to form a mycelium mass, which breaks through the periderm and invades the healthy tissue. A surface ulcer may break through from the following places, and once it expands and merges, it will cause a large area of bark rot. At the same time, the bacteria that settled in the dead tissue on the bark surface also began to move, forming necrosis points at the junction of the dead skin and healthy skin on the bark surface, and spreading the harm.

In winter, the incidence increased rapidly from 1 1 to 65438+February, and 1 reached its peak, but it spread slowly. In early spring, from February to March, the temperature rose rapidly and expanded, and it entered the peak period of rot.

Fruit trees enter the growth period after spreading their leaves and flowering, and their resistance increases, and the incidence rate drops sharply, stopping their expansion. The peak of the disease ended in May.

After the bark decays, bacteria enter the xylem under the diseased skin, and the depth can reach 0.5- 1.5 cm. After scraping the diseased skin, the hyphae in xylem can continue to survive for 3-5 years, and spread into the bark around the diseased scar, causing the diseased scar to recur.

In summer and autumn, about 80% of body surface ulcers are formed in the cortex. In winter and spring, about 50% cases are caused by the deep spread of body surface ulcers, and about 35% cases are caused by the expansion of necrotic spots in late autumn and early winter. It can be seen that the formation of bark abscission layer is closely related to the spread of rotten bacteria.

In the apple-producing areas of Liaoning, the onset process is basically the same as that of Shaanxi, but it is later in summer and spreads deeper than that of Shaanxi. The onset peak was earlier in early winter, less in1-February, and the onset peak ended later in spring.

Apple trees have antitoxin defense response to the invasion of rotten fungi, forming a barrier composed of lignified cell walls and cork layers, blocking the spread of pathogenic bacteria until the diseased tissues are eliminated. The ability of tree callus is closely related to its anti-expansion ability. The single plant or big branch with strong callus ability has strong anti-swelling ability. Callus ability is restricted by the moisture filling degree of bark. When the water filling degree of bark is above 80%, the callus ability is faster, and with the decrease of water filling degree of bark, the callus ability drops sharply. In this way, the bark of big branches with more fruits has lower water filling degree, slower healing, larger ulcer expansion area and weaker anti-expansion ability. Tree load has a significant effect on disease resistance.

Extensive cultivation management and periodic freezing injury are the main reasons for the epidemic of rot disease. Orchards with weak trees due to poor soil management, poor root growth, insufficient fertilization, excessive fruits and premature defoliation are seriously ill. On the contrary, it is lighter. In orchards with large differences in years, the disease becomes worse after the New Year. Periodic freezing injury often leads to the epidemic of rot disease. In the areas vulnerable to freezing in the north, low-lying water accumulation and green orchards in the later period are more serious. In areas with strong sunshine in winter, the large temperature difference between day and night on the sunny side of branches is also an important factor to induce rot.

Prevention and cure method

1. The key points of improving tree cultivation management and disease resistance are: soil and water conservation, improving farming, increasing fertilizer, and paying attention to the reasonable cooperation of organic fertilizer with nitrogen, phosphorus and potassium fertilizer; Reasonable pruning, thinning flowers and fruits, controlling load and balancing years; Control defoliation and pests; Control late growth, prevent petty gain and increase cold resistance.

2. Chemical control

Spraying 40% thiram wettable powder 100 times solution before fruit trees germinate in spring, and applying the same solution to the middle and lower parts of big branches before June-July. Spraying chemicals before germination of fruit trees in spring is the key to chemical control. Spraying thiram once can reduce the incidence rate by 50% in summer, autumn and the following spring. Spraying chemicals before germination in spring and applying them again in summer can achieve 80% disease prevention effect.

3. Heal in time and protect the scar.

Check carefully before early winter, and scrape off the rotten things in summer and autumn. In particular, it is necessary to carefully check the surface ulcers, thoroughly scrape off the diseased tissues on the bark surface before winter, and prevent them from spreading to the deep layer after winter to aggravate the harm. Continue to check in the spring peak period and scrape it in time.

After each curettage, 50- 100 times thiram solution should be applied to kill residual bacteria. In spring, after applying chemicals to the rotten parts of bark, it is necessary to apply thiram arsenic again in June-July in combination with branches to control the activities of xylem bacteria and prevent the recurrence of diseased scars. Applying Fu Mei arsenic twice in spring and summer can prevent the recurrence of 80% diseases, and applying Yubiqing emulsion twice or five times can prevent the recurrence of 80-90% diseases, and promote the healing of scars to some extent.

Paint it white

In areas with strong sunshine in winter, branches should be painted white before winter to prevent sunburn in winter.

5. Bridging or foot joint

Bridge and pin the scar on the main branch to help restore the tree.

6. Handle the diseased branches and residual piles in time.

Collect diseased branches and skins, remove diseased branches and stumps left on trees, and burn them in time; Pruned branches in winter should be removed from the garden in time to prevent the breeding and infection of germs.