Causes and Control Methods of Gray Leaf Spot in Corn, Apple, Chinese Bayberry, Cowpea and Soybean

Gray leaf spot, also known as brown spot, speckle disease or frog-eye leaf spot, is a worldwide disease. So what are the causes and control methods of gray leaf spot in corn, apple, Chinese bayberry, cowpea and soybean? Let's take a detailed look below.

I. Corn gray leaf spot

1. For corn planted in early to mid-May during the sowing period, corn gray leaf spot generally begins to develop in mid-to-late July, starting from the bottom leaves, developing slowly in July, damaging the middle leaves, gradually worsening in August and rapidly breaking out into an epidemic, reaching its peak in mid-to-late September and forming the so-called “autumn wind disease”. In ordinary fields, the diseased leaf rate of corn gray leaf spot reaches 99%, and the disease index reaches above 30; in severely affected fields, the diseased leaf rate of corn gray leaf spot reaches 99%, and the disease index reaches above 90, causing the entire corn plant to wither and die within a short period. The disease follows an “S”-shaped curve epidemic during the corn growth period.

2. Chemical control methods: at the current early stage of the disease, choose 10% difenoconazole granules at 1500 times dilution (40 grams per mu diluted in 60 kg of water), or choose 15% triadimefon wettable powder at 600 times dilution (100 grams per mu diluted in 60 kg of water), or choose 30% hexaconazole suspension concentrate (20 g per mu, diluted in 45 kg of water), applying once every 7—10 days, for a total of 3 applications. In areas with more severe disease, seeds can be treated with 15% triadimefon wettable powder at 0.12% of seed weight to reduce disease occurrence.

II. Apple gray leaf spot

1. There are 2 pathogens causing apple gray leaf spot: Phyllosticta pirina and Coryneum foliicola. When environmental conditions are suitable in spring, conidia are produced and spread by wind and rain. In northern fruit-growing areas, the disease begins in mid-to-late May, with the peak period from July to August. Generally, more disease occurs in autumn, and the damage is also more severe. The occurrence and epidemic of this disease are closely related to climate and variety. In years with high temperature, high humidity and early, abundant rainfall, the disease occurs earlier and more severely. The Guoguang variety is susceptible.

2. Control methods: In areas with severe disease, choose resistant varieties. Gray leaf spot mostly occurs in autumn, so efforts should focus on control in the later period. Before the disease occurs, protective agents should be the main approach, and the following agents can be used: 1:2:200 Bordeaux mixture; 200 times zinc-copper-lime solution (zinc sulfate 0.5 : copper sulfate 0.5 : lime 2 : water 200); 30% basic copper sulfate suspension concentrate at 300-500 times dilution; 70% mancozeb wettable powder at 500-600 times dilution.

III. Chinese bayberry gray leaf spot

1. This pathogen is a fungus belonging to Ascomycota, Loculoascomycetes, Dothideales, Dothideaceae.

2. Control methods: (1) In spring, prune dead branches and sweep up fallen leaves to reduce sources of disease infection.

(2) For newly planted Chinese bayberry, choose mountain land with good drainage and sufficient sunlight as much as possible, and during cultivation and management pay attention to applying more organic fertilizer and potassium fertilizer.

(3) Chemical protection and control. According to trials by the former Zhejiang Agricultural University and the Zhejiang Academy of Agricultural Sciences, it is considered appropriate to spray once in late May and once after fruit harvest, with 1:2:200 Bordeaux mixture for the former and 70% thiophanate-methyl wettable powder at 800 times dilution for the latter.

IV. Soybean gray leaf spot

1. The pathogen overwinters as mycelium or conidia on diseased residues or seeds, becoming the primary infection source the following year.

2. Control methods: (1) Choose resistant varieties Ludou No. 10, Suinong No. 8, and Heilongjiang's Hefeng No. 25, No. 27, No. 28, No. 29 and No. 30; Hefeng 34 is resistant to physiological races 1, 2, 3, 4, 7, 8, 9 and 10. However, variety resistance is very unstable, so in production close attention should be paid to changes in pathogen virulence and new resistant varieties should be replaced in a timely manner.

(2) Advocate agricultural control with rational crop rotation, avoid continuous cropping, and carry out deep plowing promptly after harvest.

(3) Spray chemicals to control disease on leaves or seeds. At the peak podding stage, use aerial spraying of 36% carbendazim suspension concentrate at 500 times dilution or 40% chlorothalonil suspension concentrate (Shuntianxing No. 1) at 600 times dilution (Beijing Shunyi County Pesticide Factory), 50% thiophanate-methyl wettable powder at 600-700 times dilution, 50% benomyl wettable powder at 1500 times dilution, 65% jiameiling wettable powder at 1000 times dilution, or 50% duomeiling wettable powder at 800 times dilution, once every 10 days or so, for 1 or 2 applications.

V. Cowpea gray leaf spot

1. It overwinters as conidial clusters or mycelium on diseased residues in the soil, and the mycelium or spores can survive on diseased residues for half a year.

2. Control methods: (1) Thoroughly remove diseased residues to reduce the primary infection source.

(2) Implement a 2-3 year rotation with non-legume crops.

(3) Use resistant varieties.

(4) Manage temperature and humidity well, pay attention to draining moisture, and improve ventilation and air permeability.

(5) At the early stage of the disease, spray 50% carbendazim wettable powder at 500 times dilution, or 75% chlorothalonil wettable powder at 700 times dilution, or 50% benomyl wettable powder at 1500 times dilution.