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 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 the first and middle ten days of May during the sowing period, corn gray leaf spot generally begins to develop in the middle and late ten days of July, starting from the lower leaves, developing slowly in July and affecting the middle leaves, gradually worsening in August and rapidly breaking out into an epidemic, reaching its peak in the middle and late ten days of September, forming the so-called “autumn wind disease”. In general 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 disease onset, choose 10% difenoconazole granules at 1500 times dilution (40 grams per mu mixed with 60 kg of water), or choose 15% triadimefon wettable powder at 600 times dilution (100 grams per mu mixed with 60 kg of water), or choose 30% hexaconazole suspension concentrate (20 g/mu, mixed with 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 dressed with 15% triadimefon wettable powder at 0.12% of seed weight to reduce disease incidence.

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 regions, disease onset begins in the middle and late ten days of 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 and abundant rainfall, disease onset is early and severe. The Ralls Janet variety is susceptible.

2. Control methods: In severely affected areas, choose disease-resistant varieties. Gray leaf spot mostly occurs in autumn, so emphasis should be placed on late-season control. Before disease onset, protective fungicides 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 experiments by the former Zhejiang Agricultural University and the Zhejiang Academy of Agricultural Sciences, it is considered appropriate to spray once in the late ten days of May and once after fruit harvest; for the former, 1:2:200 Bordeaux mixture is suitable, and for the latter, 70% thiophanate-methyl wettable powder at 800 times dilution is suitable.

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 disease-resistant varieties Ludou No. 10, Suinong No. 8, Heilongjiang's Hefeng No. 25, No. 27, No. 28, No. 29, No. 30, and Hefeng 34, which 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 disease-resistant varieties should be replaced in a timely manner.

(2) Advocate agricultural control with rational crop rotation to avoid continuous cropping, and plow deeply promptly after harvest.

(3) For chemical control of leaf or seed diseases, at the full pod-setting 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, 50% duomeiling wettable powder at 800 times dilution, once every about 10 days, 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 primary infection sources.

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

(3) Use disease-resistant varieties.

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

(5) At the early stage of disease onset, 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.