Detailed Explanation of the Damage Symptoms, Occurrence Patterns and Control Methods of Common Kiwifruit Diseases

Damage Symptoms, Occurrence Patterns and Control Methods of Common Kiwifruit Diseases

Fund Project:2016 Tai'an City Science and Technology Development Plan (2016NSOO57); Shandong Provincial Key R&D Plan (2017GNC10118).

1 Bacterial canker

Kiwifruit bacterial canker is a bacterial disease caused by Pseudomonas syringae pv. actinidiae. At present, this disease has become a devastating disease restricting the development of the kiwifruit industry. Its occurrence is characterized by a wide suitable range, rapid spread, strong pathogenicity and great difficulty in eradication, and it can cause large-scale tree death in a short period.

1.1 Damage symptoms

This disease can damage the trunk, branches, young shoots, leaves and other parts. After infection, the cortical tissue of the trunk or branches softens and swells, and later the diseased part cracks, and a milky white mucus oozes from wounds, lenticels, bud eyes, leaf scars, branch forks and other parts, later turning yellowish brown. During the bleeding period of the host, it often mixes with the bleeding sap and appears reddish brown or rust red, and it is infectious. When the cortex is peeled back, the phloem can be seen to be rotted, the xylem is dark brown, and the diseased tissue sinks and rots in an ulcer-like manner. In severe cases, the whole plant is infected and the tree dies. After young branches are infected, the young leaves on them curl, and the flower buds wilt and cannot open. After leaves become diseased, red small spots first form, and a yellow halo of 2-5 mm can be seen around them, later forming irregular or polygonal brown lesions. The lesions on the back of the leaves later become consistent with those on the leaf surface, but the color is darker. When the exudate dries and loses water, it appears scaly and lumpy, and the damaged leaves easily fall off.

1.2 Occurrence patterns

The pathogen mainly overwinters in diseased tissues of canes and vines, and can also overwinter in the soil with diseased residues. In spring, bacteria ooze from the diseased parts and are spread by wind, rain, insects, or during agricultural operations by pruning shears, farm tools, etc., and invade through stomata, water pores, lenticels, wounds (insect wounds, frostbite, knife wounds), etc. After the pathogen invades the cell tissues, after a period of reproduction, it destroys the conducting tissues and mesophyll tissues, and oozes bacterial pus for secondary infection.

Low temperature and high humidity favor disease occurrence. Low temperature and frequent rain in early spring, and a long period of freezing damage in winter, both aggravate the disease. When the average temperature over a ten-day period in spring is 10-20°C, it is favorable for the spread of the disease, and the most suitable temperature for disease occurrence is 15°C. When the temperature exceeds 25°C, the spread stops and no new lesions appear. Throughout the entire growth period of kiwifruit, the disease is relatively common during the spring bleeding period and then becomes more severe. In addition to serious damage to the main trunk, the incidence on main branches and lateral branches increases rapidly. After the petal-fall stage, the spread of the disease gradually slows down, and as the temperature rises, the lesions basically stop spreading.

1.3 Control methods

① Strengthen quarantine, do not transfer propagation materials from diseased areas, and immediately destroy diseased plants when found. ② Reduce wounds on the tree body and lower infection by the pathogen. ③ Strengthen soil, fertilizer and water management and cultivate robust trees. ④ Disinfect pruning shears. After pruning diseased trees, the same shears must not be used immediately to prune healthy trees; they can only be used after strict disinfection, and alcohol or fire can be used. ⑤ When the disease occurs in spring, if reddish brown bacterial pus is found flowing out, scrape it off immediately. When scraping to the junction of diseased and healthy tissue, the scraper must be disinfected before continuing to scrape. After scraping clean, apply 50-fold lü rutong, or apply lime sulfur stock solution plus 72% agricultural streptomycin. The scraped diseased bark must be collected and burned, and must not remain in the orchard. ⑥ Grasp 3 key control periods: after fruit harvest or before winter, after the Beginning of Spring to before bud break, and after bud break to the petal-fall stage. Optional agents include 95% CT original powder, agricultural streptomycin, junduqing, copper hydroxide, etc. ⑦ Thoroughly clean the orchard. After winter pruning, collect and treat or burn the pruned dead branches, residual leaves and residual fruits in a centralized manner to reduce the spread of the pathogen.

2 Anthracnose

Anthracnose is a fungal disease that occurs in all producing areas. It mainly damages leaves and fruits, and can also infect branches. Foreign reports suggest that the main pathogens causing kiwifruit anthracnose include Colletotrichum gloeosporioides, C. acutatum, Glomerella cingulata, etc. Some domestic scholars believe that the pathogen of this disease is G. septospora, and that this pathogen can cause disease on inoculated kiwifruit fruit stalks only through wound inoculation.

2.1 Damage symptoms

Diseased leaves often develop grayish brown lesions starting from the edge. They are initially water-soaked, with a clear boundary between diseased and healthy tissue, and gradually turn into irregular brown spots. Later, the center of the lesion turns grayish white and the edge is dark brown, with many small black dots scattered on it (that is, the pathogen's acervuli or perithecia). Some lesions rupture in the middle to form holes, and the leaf margin curls slightly toward the back of the leaf. In wet and rainy conditions, the leaves rot and fall off. After fruits are damaged, round brown lesions form, the surface is slightly sunken, and small granular dots appear on the lesions later. If the weather is humid, pink mucus oozes out. After canes and vines are infected, light brown small spots first form, the area around the lesion is brown, and later it expands into an oval shape.

2.2 Occurrence patterns

The pathogen overwinters as mycelium and acervuli in diseased residues or in parts such as bud scales and axillary buds. The next spring, the pathogen produces conidia, which are spread by wind, rainwater, insects, etc., and invade through stomata, wounds or the epidermis, with latent infection. Kiwifruit orchards with high temperature and high humidity have severe occurrence, and it is likely to occur mixed with other diseases.

2.3 Control methods

① Strengthen cultivation management. Prune reasonably, apply organic fertilizer, and enhance tree vigor. ② In combination with winter pruning, cut off diseased canes and mummified fruits, and remove overwintering pathogens. ③ Chemical control, with prevention as the main approach. Optional agents include: Bordeaux mixture, 70% zineb wettable powder, 50% carbendazim wettable powder, 30% difenoconazole suspension concentrate, etc. Spray once every 10-15 days, for 2-3 consecutive sprays, which can prevent the occurrence and spread of this disease.

3 Brown spot disease

Brown spot disease is an important leaf disease of kiwifruit. Domestic reports suggest that Alternaria alternata (Fr.: Fr.) Keissler is the pathogen causing brown spot disease in Actinidia chinensis.

3.1 Damage symptoms

It mainly damages leaves and branches, and sometimes also damages the fruit surface. Lesions mainly begin at the leaf margin, and also occur on the leaf surface. Initially, water-soaked dirty green small spots appear, then spread along the leaf margin or inward, forming irregular brown lesions. Under rainy and high-humidity conditions, the disease develops rapidly, and the lesions change from brown to black, with a clear boundary between diseased and healthy tissue. Under normal climate, the lesion is dark brown around the edge, brown to light brown in the center, with many black small pycnidia scattered or densely borne on it, that is, the pathogen's pycnidia. Under high temperature, damaged leaves curl toward the leaf surface, easily rupture, and later dry up and fall off. Lesions in the middle of the leaf surface are noticeably smaller than those at the leaf margin, and the lesions show through the back of the leaf as yellow-brown.

3.2 Occurrence patterns

The pathogen overwinters in the form of conidia or mycelium in diseased residues. The next spring, it splashes onto young leaves with wind and rain for primary infection and multiple secondary infections. Generally, the disease begins in May-June, enters its peak period in July-August, and if there is frequent rain in September, the disease is severe. High temperature and high humidity favor disease occurrence.

3.3 Control methods

① Thoroughly clean the orchard in winter. Collect the pruned canes and fallen leaves out of the orchard and burn them. Deeply plow the orchard surface soil 10-15 cm so that diseased residual leaves and scattered pathogens on the soil surface are buried in the soil, reducing overwintering pathogens. ② Pay attention to shaping and pruning so that the orchard is ventilated and lets light through. ③ Carry out chemical control in the early stage of disease occurrence. Optional agents include 1:1:100 Bordeaux mixture, 80% mancozeb wettable powder 1000-fold, 50% thiophanate-methyl or 70% thiophanate-methyl 1000-fold solution, etc. Spray once every 10-15 days, for 3-4 consecutive sprays, which can control the occurrence and spread of the disease. After orchard cleaning is completed, 5-6°Be lime sulfur mixture can also be sprayed to kill pathogens and mites on the vines.

4 Gray mold

Gray mold mainly occurs during the flowering stage, young fruit stage and storage period of kiwifruit. This disease is caused by Botrytis cinerea Persoon, and the sexual stage of this pathogen is Botryotinia fuckeliana.

4.1 Damage symptoms

It mainly damages flowers, leaves and fruits. After flowers are infected, the flowers turn brown and rot and fall off, and a large amount of gray mold layer forms on the surface. In the early stage of young fruit infection, water-soaked spots form, then expand to the whole fruit. When humidity is high, grayish white moldy material appears on the skin of diseased fruits. After diseased flowers fall onto leaves, they cause white to yellow-brown lesions on the leaves, and grayish white moldy material (that is, the pathogen's conidiophores and conidia) often appears when humidity is high. Fruits in the early growth stage are relatively disease-resistant, and field rot is uncommon, but fruits that have already been infected will develop disease quickly in cold storage and infect other fruits.

4.2 Occurrence patterns

The pathogen overwinters as mycelium on diseased parts or rotten diseased residues, or as sclerotia in the soil. When conditions are suitable, it produces spores, which are spread by airflow and rain splash. From the bud break stage to the petal-fall stage of kiwifruit is the high-incidence period of gray mold. This pathogen can grow normally within 5-30°C, and the optimum growth temperature is about 20°C. Continuous high humidity, insufficient sunlight and poor ventilation easily lead to disease occurrence; long retention time of moisture leads to severe disease.

4.3 Control methods

① Strengthen soil, fertilizer and water management to enhance tree disease resistance. ② Pay attention to orchard drainage to prevent water accumulation in the orchard. ③ Prune reasonably, remove diseased fruits in time, and take them out of the field for deep burial, maintaining good ventilation and light transmission in the orchard. ④ Grasp the key control period before flowering. Optional agents include 50% carbendazim wettable powder 800-fold, 75% chlorothalonil wettable powder 600-fold, vinclozolin wettable powder 1000-fold, 50% iprodione wettable powder 800-1000-fold solution, etc. Note to alternate medications. ⑤ Avoid and reduce fruit injury during harvest.

5 Sclerotinia rot

The pathogen of kiwifruit Sclerotinia rot is Sclerotinia sclerotiorum (Lib.) de Bary, which can parasitize more than 300 kinds of plants such as rapeseed, eggplant, tomato, lettuce, pepper, potato and clover.

5.1 Damage symptoms

It commonly damages flowers and fruits. When male flowers are damaged, they are initially water-soaked, then soften, and then decline and wither in clusters, shrinking dry into brown masses. When female flowers are damaged, the flower buds turn brown and wither and cannot open. Under rainy conditions, white moldy material grows on the diseased parts. When fruits are damaged, water-soaked chlorotic patches first appear, the diseased part sinks, and it gradually turns into soft rot. Severely diseased fruits in the field generally fall off one after another. A small number of fruits, due to flesh rot, rupture of the peel, and overflow of rotten juice, become shriveled, and later irregular black sclerotia form on the surface of the peel of diseased fruits. Diseased fruits are not resistant to storage and transport and are prone to rot.

5.2 Occurrence patterns

Mycelium multiplies in large amounts in diseased fruits and forms sclerotia, and the sclerotia overwinter in the soil as diseased residues fall to the ground. In the next spring at the beginning of the flowering stage of kiwifruit, the sclerotia germinate, produce apothecia and eject ascospores, which are spread by wind and rain and invade the flower organs of kiwifruit to cause primary infection. A small number of ungerminated sclerotia in the soil can germinate continuously and infect growing fruits, causing fruit rot. When the temperature reaches 20-24°C and the relative air humidity is 85%-90%, the disease develops rapidly. If spring is warm and rainy, it is favorable for sclerotial germination, many ascospores are produced, and the disease is severe.

5.3 Control methods

① Thoroughly remove the pathogen. Remove diseased fruits at any time and take them out of the orchard for deep burial. After autumn fertilization, plow and bury the surface soil 10-15 cm so that sclerotia on the soil surface are buried deep in the soil and cannot germinate and infect. ② Chemical control. Grasp the 3 control periods of before flowering, petal-fall stage and young fruit stage. Optional agents include 50% vinclozolin wettable powder 1000-fold, 40% dimethachlon wettable powder 1000-fold, 70% thiophanate-methyl wettable powder 1000-1500-fold, 50% procymidone wettable powder 1000-fold solution, etc.