What is pear scab?

symptom

Pear scab is also called scab and fog disease. Damage to leaves, fruits, buds, inflorescences, new shoots, petioles, fruit stalks and other parts. The main feature of the disease is the formation of black mold layer in the affected area.

When leaves are infected, round or amorphous lesions usually appear along branches or midvein on the back of leaves without obvious edges. A few days later, a soot-like layer of black mold appeared, namely, conidiophore and conidia of pathogenic bacteria. In severe cases, the lesions spread to the surface of the leaves, and when there are many lesions on the leaves, the leaves turn yellow, red or red-green. Petiole is also prone to disease. Most of the diseased leaves died and fell off in the early stage.

After the young fruit was infected, a small yellowish round lesion appeared at first, which gradually expanded to 5-10 mm. After about 3-8 days, a layer of black mold appeared on the lesion. However, some early lesions did not expand and did not produce black mold, but became dark green spots. The pulp of the affected part is hard and bitter. The diseased tissue stopped growing and developed, and the diseased spots gradually drooped and cracked with the increase of fruits. In severe cases, the diseased fruits are uneven and easy to fall off early. The black mold on the lesion is often washed away by rain in the later stage of the disease. Due to saprophy of other molds, gray-white or pink mold layers often appear in the affected areas. When the fruit is killed in the later stage, the diseased spots are yellow, slightly concave and generally not cracked.

After the bud body is damaged, there are many hairs on the scales, and black mold is produced on the surface in the later stage, which can cause the bud scales to crack and die in severe cases. The inflorescence and base of new shoots extracted from diseased buds are the first to get sick. Inflorescence diseases mostly occur at the base and pedicel, which often leads to tissue death above the diseased part. The damage of new shoots is also covered by purple-black mold layer at the base. In the late stage of the disease, it gradually droops and cracks, becoming an ulcer spot. The young parts of new shoots are also prone to disease, and the long branches are the most affected. The lesion was oval, pale yellow at first, bean-sized, and slightly raised. There is also a layer of black mold on the surface of the lesion, and then the lesion gradually sinks and cracks.

Pathogen (Figure 1-24)

Fig. 1-24 pear scab

Conidiophore 2 conidia 3 ascospores and 4 ascospores. False capsule shell.

The asexual reproductive stage of pear scab is pear scab. Fuckel, most of conidiophore are 3- 10 branches, which are dark brown, single-celled, erect or curved, often unbranched, with many obvious spore marks, dentate or verrucous protrusions on the stalk wall, and are 10-20μ m long. Conidia are dark brown, ovoid to spindle-shaped, with a slightly sharp end and a single terminal on the verrucous process of spore stalk, with a size of 7. .. in Shaanxi Wugong, Liaoning Luda, Hebei Shijiazhuang and Beijing, sexual stages have been found on fallen leaves. According to Zhang Yi Tanaka's research (1964), the pear scab fungus infecting western pears is completely different from the pear scab fungus infecting China pears and Japanese pears in culture characters, pathogenic forms, pathogenicity and harmful symptoms, and the latter is named Venturia nashikola Tanaka et Yamamoto. Daughters are clustered or scattered under the epidermis of diseased leaves, which are oblate, conical or spherical. The bottom of the pseudocapsule shell is slightly flat and black, with a diameter of 52.5-138.7 μ m. There are no bristles around the orifice, and the shell wall is black and leathery, consisting of 2-3 layers of cells. Ascomycetes are colorless, rod-shaped or oval, and are 35-60μ m long. There are 8 ascospores in the ascus. Ascomycetes are light brown, diploid and sole-shaped, with the size of 10- 15× 3.5-6.3 microns, and the average micron is12.63× 5.0 μ m. Conidia and ascospores are slightly smaller than those of western pear scab.

Occurrence law

Pathogens mainly overwinter on bud scales, diseased leaves, diseased fruits and branches with conidia and hyphae. When the temperature and humidity conditions were suitable in the following spring, the remaining overwintering conidia and the newly formed conidia in the epidemic area spread and infected by wind and rain. In some areas and years, if there is a lot of rain in autumn and it is warm and humid in winter, ascocarp can be formed on the diseased leaves. Ascomycetes mature in spring and emit ascospores when it rains, spreading infection. In Liaoning, Beijing and other places, conidia remaining on bud scales, diseased branches and diseased buds can survive the winter and are the main sources of infection in the following year. In Wugong area of Shaanxi Province, in dry and cold winter, the pathogen mainly overwinters with conidia left on fallen leaves, while in warm and humid winter, a large number of ascomycetes can be formed. In individual years, ascospores play a major role in spreading infection. From the local situation, in different years and regions, germs can overwinter in different ways.

Conidia have strong vitality. At the temperature of-65438 04℃, the germination ability is still above 50% after 3 months, and the survival period on diseased leaves can be as long as 4-7 months. The temperature range of conidia formation is 12-20℃, and the optimum temperature is 16℃. The germination temperature of conidia is 5-30℃, and the optimum temperature is 22-23℃. The relative humidity required for conidia germination is above 70%, the germination rate is the highest above 80%, and it will not germinate below 50%. Conidia can germinate within 4-5 hours under suitable temperature and humidity conditions. The mycelium can grow at 7-30℃, but 20℃ is the most suitable. The incubation period of the pathogen in Guanzhong area of Shaanxi Province is 12- 19 days, and the higher the temperature, the shorter the incubation period. In Xingcheng, Liaoning, it takes 20-25 days in early May and 14-20 days from late June to early July. The incubation period from April to May in Yanxian County, Shanxi Province is about 15 days. It takes only 5-48 hours for conidia to germinate and invade pear tissue. In areas where ascospores overwinter, such as spring drought, ascospores cannot spread by radiation. Even if it rains again in the future, it can spread, but the germination rate is also reduced.

Pear scab first appeared in inflorescence and the base of new branches of diseased buds. When new branches and flower clusters are diseased, they become reinfection centers and gradually spread to fruits, leaves and shoots. The disease occurred in the epidemic year, from flowering to harvest. If infected before harvest, disease spots will appear during storage. Inflorescence diseases mostly occur at the base of inflorescence and pedicel. The infection of new buds is also mostly at the base. Petiole disease can gradually spread up to the leaves along the petiole. Young leaves and young fruits are susceptible to diseases, and a large number of leaves and fruits can be dropped when the diseases are serious. After leaf spreading 1 month, the leaf tissue is aging, the disease resistance is enhanced, and the damage is light. The occurrence and prevalence of this disease are closely related to rainfall. In early spring rain, germs invade early and can infect buds and young leaves. Drought delays the onset and only infects leaves and fruits. Due to the different rainfall time in the morning and evening of each year, the invasion time and initial infection site of bacteria are also different. Generally speaking, July-August is the rainy season and the peak of disease occurrence. The disease is serious in rainy years and light in dry years. In Henan, northern Jiangsu and northern Anhui, the disease began in early and middle April. During April-May, if there are many rainy days, the disease will develop rapidly and enter the peak period. Especially when it rains continuously, the disease is particularly serious, forming the first peak of the disease. If there is little rain from June to mid-July, the disease can stop developing once. From mid-July to the end of August, usually the rainy season, the second peak appeared again. The disease not only infects fruits and leaves, but also overwintering buds, and becomes the primary source of infection in the coming year. Due to different climatic conditions, the occurrence period of the disease is quite different in different regions of China. In Yunnan, Guangdong, Guangxi and other regions, the disease usually begins at the end of March, with the peak in June-July, and gradually declines after mid-August. In the Yangtze River basin, the disease usually begins in early April, and the rainy season reaches its peak in May-June. Hebei, Shanxi, Shaanxi and other regions, generally from the middle and late April, July-August is the peak of the disease. In Liaoning, Jilin and other areas, the onset usually begins in the middle and late May, and the peak of the onset is August.

Different pear varieties have different disease resistance. The varieties with serious diseases are Yali pear, Jing pear, Huangli pear, QiuPak Lei pear, Jianba pear, Gai Hua pear, Yuanba pear, Mali pear, Zhuzui pear and pear, followed by Dangshan Baisu pear, Laiyanggu pear, Jincunqiu pear, Yue Ming pear, Kangde pear, Li Hongli pear, Xiao Hongli pear, Dongguo pear and Changshilang pear. Disease-resistant varieties include Xiangshui pear, Boli pear, Honey pear, Sydney, Huangpi pear, Zaosu, Jinfeng and Xiyang pear (Bali).

Prevention and cure method

1. Reduce the source of overwintering bacteria

Carefully clean up fallen leaves in late autumn, remove diseased fruits and dead branches, and reduce the source of overwintering bacteria.

2. Eliminate the infection center of germs.

In the early stage of the disease, the diseased flowers and disease tips should be removed in time to eliminate the infection center of the bacteria and prevent the spread of the bacteria.

3. Chemical control

In southern China, the onset time is early, and it can be seriously damaged in April-May. The suitable time for spraying control should be before flowering and when the flower drop is about 70%, spraying 1 times 1: 2: 240 bordeaux mixture to protect inflorescences, shoots and new leaves. In the future, according to the rainfall situation, 15-20 days will be continuously sprayed with bordeaux mixture 1: 2: 200 to protect leaves, branches and fruits. The susceptible varieties can be changed to 600 times of 65% zineb wettable powder. In the northern pear region, in rainy years, the first application should be in mid-May, that is, after the sepals of Pak Lei fell off, at the early stage of the disease tip, spraying Bordeaux solution 1: 2: 240, the second in mid-June, the third in late June to early July, and the fourth in early August. If there is less spring rain, there is no need to spray the medicine for the second time. For varieties with late onset, such as Gai Hua pear and Anli pear, the first and second drugs may not be sprayed. Except bordeaux solution and zineb, 70% thiophanate methyl WP 800- 1000 times solution, 75% chlorothalonil WP 800- 1000 times solution, 50% carbendazim WP 1000 times solution or 45% mancozeb aqueous solution 65438+. In order to enhance the adhesiveness of the liquid medicine and reduce the rain erosion, it is necessary to add 1/3000 leather glue or 1% bean juice, or 650 1 liquid medicine and 0. 1% spreading agent for each spraying.