Symptoms and Control of Powdery Mildew, a Common Strawberry Disease

Schematic diagram of strawberry powdery mildew

As is well known, the strawberry is a fruit and vegetable plant that is both ornamental and edible. Thanks to advanced cultivation techniques in recent years, this plant that grows on the ground can now grow independently in flower pots. With proper care and management, it can still bloom and bear fruit, but there is always a "but." Without enough growing experience, strawberries are very susceptible to powdery mildew, so what should you do when a strawberry is infected with powdery mildew?

I. Causes of Strawberry Powdery Mildew

The onset of strawberry powdery mildew is related to temperature and humidity. Generally, low temperature and high humidity produce powdery mildew disease. The onset temperature is about 12°C-23°C. In autumn or spring, continuous overcast, rainy, or foggy weather makes infection with powdery mildew likely. Few sunshine hours, low temperature, and high humidity also make infection by the pathogen likely. Continuous cropping in greenhouses leads to more severe disease. Fertilization should involve more phosphorus and potassium fertilizer and less nitrogen fertilizer, which can reduce the incidence of powdery mildew. High-resistance varieties should be selected for strawberry cultivation. The highly resistant varieties for strawberry powdery mildew should be chosen as follows: Tudla, Hongshimei, Xingxiang, Fengxiang, and Darselect.

II. Symptoms of Strawberry Powdery Mildew

Powdery mildew mainly damages strawberry leaves, petioles, or fruit. In the early stage of the disease, thin white mycelium grows on the underside of the leaves. In the later stage, the mycelium becomes dense and powdery. If not properly controlled, it will gradually worsen and the entire leaf will wither and die. Flowers and flower buds will be damaged to varying degrees, pollination will be poor, young fruit cannot grow normally, and the fruit surface will have a layer of white powdery substance. The fruit grows slowly and hardens and cannot be eaten. The disease spreads quickly and should be controlled promptly. The control of strawberry powdery mildew in greenhouses must adhere to the strategy of "prevention first, whole-process control." Only by implementing comprehensive supporting measures such as agricultural practices and chemical agents can good results be achieved.

III. Control Methods for Strawberry Powdery Mildew

1. Select disease-resistant varieties

Relatively disease-resistant varieties such as Mingbao and Yixiang can be selected, and susceptible varieties such as Fengxiang should be avoided as much as possible.

2. Implement crop rotation and replanting

In heavily diseased fields, rice-upland crop rotation can be carried out. Crops suitable for rotation are cruciferous vegetables and legumes. Solanaceous crops share common diseases with strawberries and are not suitable as rotation crops.

3. Clean the field

After strawberry harvest, promptly clean up and burn diseased plant residues. Before planting, remove all kinds of weeds from the field. During the growing period, promptly remove diseased leaves, diseased fruit, and old leaves, and take them out of the greenhouse to be burned or deeply buried in a centralized manner, so as to avoid re-infection of the pathogen with rainwater and airflow and reduce the base number of inoculum.

4. Strengthen cultivation management

Before transplanting, the strawberry field should be deeply plowed and the soil turned, and disinfection should be carried out for the greenhouse, soil, and transplanted seedlings to reduce the primary inoculum source. Weeds should be removed before mulching to reduce the role of weeds as intermediate hosts of powdery mildew. Cultivate strong seedlings and transplant at the right time. Plant at a reasonable density and ensure suitable plant and row spacing. Ensure good ventilation and air exchange in the greenhouse, and pay attention to timely drainage after rain to reduce field humidity. Fertilize reasonably. Base fertilizer should mainly be decomposed organic fertilizer and phosphorus and potassium fertilizer, and topdressing should mainly be nitrogen-phosphorus-potassium compound fertilizer. Avoid partial or excessive application of nitrogen fertilizer, and improve plant disease resistance. Use water scientifically, pay attention to drainage after rain, reduce field humidity, and reduce pathogen infection. After budding and during the fruit enlargement stage and peak harvest period, special attention should be paid to water management. It is best to use drip irrigation under film, and flooding irrigation must be avoided. Reform the cultivation system and implement a one-year-one-planting system.

Strawberry

5. Ecological control

Regulating the temperature and humidity in the greenhouse can effectively control the occurrence of powdery mildew. First, adopt the techniques of double-row ridging and transplanting with full-film mulching, which helps increase ground temperature, reduce the temperature difference with the greenhouse, reduce condensation, and enhance light. Second, establish facilities for drip irrigation or under-film watering. If there are no under-film watering facilities, water should be used scientifically and flooding irrigation should be avoided, with the principle of "better dry than wet." Ventilation should be strengthened to reduce the relative humidity in the greenhouse. The ideal relative humidity in the greenhouse is about 60% during the flowering and fruit-setting period and about 70% during the fruit enlargement period.

6. Chemical control

1) Timely application of chemicals. The control of strawberry powdery mildew should adhere to prevention first and whole-process control, starting from the seedling stage. During the strawberry runner propagation and seedling stage, promptly remove weak seedlings and diseased seedlings, and apply chemicals for prevention 2~3 times. After planting, focus on controlling the central diseased plants and surrounding plants, with continuous control 2~3 times at an interval of 7-10 days.

2) Use the right chemicals for the disease. The following can be selected: 25% triadimefon wettable powder 2000 times solution, 75% chlorothalonil wettable powder 600 times solution, 10% difenoconazole water-dispersible granules 1000-1500 times solution, 12.5% myclobutanil emulsifiable concentrate 1500-2000 times solution, 50% tebuconazole wettable powder 800 times solution, 50% sulfur-multisulfide suspension concentrate 500~600 times solution, 4% agricultural antibiotic 120 aqueous solution 600~800 times solution. For chemical application during continuous rainy weather, Baijun smoke fumigant can be selected.

3) Scientific application. Spray evenly and thoroughly, covering both the front and back of leaves, and pay attention to alternating the use of control agents to prevent resistance.

4) Safe application. Choose safe, efficient, and low-toxicity chemicals for control. Greenhouse strawberries, especially after the greenhouse is covered, are very sensitive to chemicals. Various chemicals should be sprayed at the lower limit concentration, and they must not be mixed with other chemicals or micro-fertilizers to avoid phytotoxicity. Generally, no chemicals should be used during the flowering period, otherwise malformed fruit is likely to occur. From the fruit enlargement stage to the harvest stage, pollution-free biological pesticides should be selected for control. Strictly implement the safe interval period for pesticides. Generally, chemical application should stop 7d before harvest to control pesticide residues.

If the disease is discovered early, the cause is identified, and the right treatment is applied, strawberries can still continue to bloom and bear fruit. In short, if you grow strawberries according to their growth requirements and also do basic care work in normal times, then under normal circumstances strawberries will not be infected by diseases.

What Common Pests and Diseases Do Strawberries Have?

Strawberries are mainly eaten fresh, and no pesticides can be used during fruit ripening. Therefore, in strawberry production, the principle of prevention first and comprehensive control must be carried out. There are many kinds of strawberry pests and diseases. Among the more serious diseases are strawberry gray mold, powdery mildew, and snake-eye disease. Among the more serious pests are red spider mites, aphids, and white grubs, which seriously affect the quality and yield of strawberries. The following introduces common strawberry pests and diseases and their control methods.

Strawberry Gray Mold

Damage symptoms: Strawberry gray mold mainly damages flowers, leaves, and fruit, and also infects leaves and petioles. The disease mostly begins at the flowering stage. The pathogen first infects from flowers that are about to fade or from relatively weak parts, causing the flowers to become light brown, necrotic, and rotten, and producing a gray mold layer. Leaves are mostly invaded from the edges of old yellow leaves at the base, forming a "V"-shaped yellow-brown spot, or infected along the places where petals fall, forming nearly circular necrotic spots with inconspicuous concentric rings and relatively sparse gray mold on them. Infected fruit mostly starts from residual petals or from parts near or touching the ground. It initially appears water-soaked, gray-brown, and necrotic, then the color deepens, the fruit rots, and a dense gray mold layer forms on the surface. When petioles are affected, they become light brown, necrotic, and shriveled, with sparse gray mold on them.

Control methods:

1. Select disease-resistant varieties.

2. Agricultural control. Improve cultivation management and implement crop rotation. Deep plow before planting to reduce inoculum sources. Adopt high-ridge and plastic-film mulching cultivation to prevent fruit from directly touching the ground. Choose plots with high, dry terrain and good ventilation. Plant at a reasonable density. Avoid excessive application of nitrogen fertilizer. Strengthen field management, promptly remove old leaves, dead leaves, diseased leaves, and diseased fruit, and take them out of the garden to burn or deeply bury to reduce infection sources, and use drip-free greenhouse film.

3. Chemical control. Before planting, spread 25% carbendazim wettable powder 75~90 kg per hectare, evenly rake it into the soil, eliminate residual inoculum sources, and disinfect the soil. In the early stage of the disease, the following can be used for spraying: Shijiale 40% suspension concentrate 1500 times solution, 50% vinclozolin wettable powder 1200 times solution, 65% jia-meiling wettable powder 600 times solution, 5% duo-meiling wettable powder 800 times solution, 50% anilazine wettable powder 400 times solution, 45% thiabendazole suspension concentrate 800 times solution, 10% polyoxin wettable powder 600 times solution. Spray once every 7~10 days, 3~4 times in total, focusing on flowers and fruit. In protected cultivation, choose 6.5% jia-meiling dust 1 kg/mu for dusting, or use 20% thiabendazole smoke agent 0.3~0.5 kg/mu for fumigation, for a more ideal control effect.

Strawberry Powdery Mildew

Damage symptoms: Mainly damages leaves, petioles, flowers, flower stalks, and fruit. When leaves are infected, in the initial stage white nearly circular star-shaped small powdery spots appear on the underside of the leaves, then expand outward into continuous powdery white spots with indistinct edges. In severe cases, the entire leaf is covered with white powder, the leaf edges curl upward and deform, and finally the diseased leaves gradually turn yellow and wither. When flowers and flower buds are infected, the petals become pink, and the flower buds cannot open. When fruit is infected, young fruit cannot expand normally, and later the fruit surface is covered with a layer of white powder and colors poorly. The optimum temperature for pathogen infection is 15~25°C. Below 5°C or above 35°C is unfavorable for disease onset. When relative humidity is above 80%, it can also germinate under low humidity. Especially when high temperature and drought alternate with high temperature and high humidity, and there is a large amount of inoculum, it is likely to become epidemic.

Control methods:

1. Select disease-resistant varieties according to local conditions.

2. Agricultural control. Plant at a reasonable density and maintain good ventilation and light conditions. Drain promptly after rain and strengthen water and fertilizer management. Cultivate robust plants. Strengthen field management, promptly remove old leaves and diseased leaves, and gather dead branches and residual leaves in the field and at the field edges to deeply bury or burn in a centralized manner.

3. Chemical control. In the early growth stage of strawberries, before infection with powdery mildew, 80% mancozeb wettable powder 800 times solution or 75% chlorothalonil wettable powder 600 times solution can be used for protective spraying.

In the early stage of powdery mildew onset, apply chemicals promptly for control. The following can be selected: 15% triadimefon wettable powder 1500~2000 times solution, 50% carbendazim wettable powder 500 times solution, 70% thiophanate-methyl wettable powder 800 times solution, 40% sulfur-multisulfide suspension concentrate 500 times solution, 10% difenoconazole wettable powder 3000 times solution, 40% DuPont Fusilade emulsifiable concentrate 8000~10000 times solution, 50% shibaogong wettable powder 2000 times solution, 47% kasugamycin-copper wettable powder 600 times solution.

At present, the most effective control method is to use a sulfur fumigator.

Strawberry Snake-Eye Disease

Strawberry snake-eye disease, also known as strawberry white spot disease or leaf spot disease, is widely distributed and occurs extensively in strawberry cultivation areas in China. In severe cases, the incidence can reach 40%~60%.

Damage symptoms: Mainly damages leaves, mostly occurring on old leaves. Petioles, fruit stalks, calyxes, and berries can also be affected. When leaves are infected, small irregular red to purplish-red spots first form, then gradually expand into circular or oblong spots 2~5 mm in diameter. The center of the spot is gray-white and the edge is purplish-brown, resembling a snake eye. In the later stage, many small black dots form on the spots. When the damage is severe, many spots merge into large spots and the leaves wither and die. After fruit stalks and calyxes are infected, irregular yellow-brown to dark-brown spots with darker edges form, and when dry they easily break off from the diseased part. When fruit is infected, the seeds on the berries are damaged singly or in patches, and the damaged seeds together with the surrounding fruit flesh turn black. The pathogen prefers a humid environment, and the optimum temperature for the pathogen is 18~22°C. Continuous cropping, poor drainage, extensive management in wet plots, or fields with weak plant growth suffer more severe disease.

Control methods:

1. Select disease-resistant and disease-tolerant varieties according to local conditions.

2. Agricultural control

Practice reasonable crop rotation, control the application of nitrogen fertilizer to prevent excessive growth, plant appropriately sparsely, remove old leaves and dead leaves, improve ventilation and light conditions, and promptly clean the field after harvest, gathering residual and diseased leaves to burn in a centralized manner.

3. Chemical control

In the early stage of the disease, spray or drench with 50% copper bis(dodecylbenzenesulfonate) (DT) wettable powder 500 times solution, 65% zineb wettable powder 350 times solution, 77% copper hydroxide wettable microparticle powder 500 times solution, 50% anilazine wettable powder 500 times solution, 70% thiophanate-methyl wettable powder 800 times solution, or 40% flusilazole emulsifiable concentrate 5000 times solution, once every 10 days, 2~3 times in total.

Red Spider Mite

Damage characteristics: Also known as carmine spider mite or citrus red mite. It is an important pest in protected strawberry cultivation and is widely distributed across the country. It has a broad diet and uses adult mites and nymph mites to pierce and suck plant sap on the underside of leaves. When the population is large, the leaves become gray-white, growth stops, and silk webs form on the plants. In severe cases, the leaves scorch and fall off, and the strawberries look as if burned by fire. The older the strawberry leaves and the higher the nitrogen content, the more likely red spider mites are to occur. With extensive management, plant vigor weakens and damage worsens.

Control methods:

1. Agricultural control. Plow the field in early spring, remove weeds in the field and at the field edges, and reduce pest sources. After vegetable harvest, remove dead branches and fallen leaves and burn them in a centralized manner. During strawberry seedling cultivation, promptly remove insect-infested leaves, old leaves, and withered yellow leaves, and burn them in a centralized manner.

2. Chemical control. The following can all achieve ideal control effects: 40% dicofol emulsifiable concentrate 1000~1500 times solution, 20% hexythiazox wettable powder 2000 times solution, 15% pyridaben emulsifiable concentrate 2000 times solution, 1.8% abamectin emulsifiable concentrate 6000~8000 times solution, etc.

Aphids

Damage characteristics: They occur in many strawberry production areas across the country. At the early flowering stage when strawberries bolt, large numbers of aphids migrate into strawberry fields and gather on young leaves, petioles, the underside of leaves, tender growing points, inflorescences, and flower buds, where they suck sap, causing tender buds to shrink, young leaves to wrinkle and curl and become deformed, and leaves to fail to unfold normally. Aphids are the main vector for spreading viral diseases, and the losses they cause are far greater than the losses caused by their own damage.

Control methods:

1. Spray pesticides. Chemical control is currently the most effective measure for controlling aphids. Controlling aphids can also effectively prevent viral diseases. Aphids have many natural enemies, including ladybugs, lacewings, hoverflies, and parasitic wasps. Broad-spectrum pesticides should be used as little as possible to protect natural enemies. The main chemicals include 50% menazon emulsifiable concentrate 2500 times solution, 10% imidacloprid wettable powder 4000~5000 times solution, 50% pirimicarb wettable powder 2000~3000 times solution, 20% fenvalerate emulsifiable concentrate 2000 times solution, 2.5% deltamethrin emulsifiable concentrate 2000~3000 times solution, 2.5% kungfu (cypermethrin) emulsifiable concentrate 3000~4000 times solution, etc.

2. Yellow board trapping of aphids. Winged adult aphids have a strong tendency toward yellow and orange-yellow, so set yellow boards around strawberry fields. That is, coat a 30×50 sq cm plastic film fully with orange-yellow, then coat another layer of sticky machine oil on the outside, insert it into the field or hang it 0.5 m above the ground, placing 1 board every 3~5 m. In this way, large numbers of winged aphids can be trapped and killed.

3. Silver film to repel aphids. Taking advantage of the repellent effect of silver-gray on aphids, silver-gray plastic film can be used as mulch to prevent aphids from migrating into strawberry fields.

4. Use natural enemies. Efficient and low-toxicity insecticides should be selected, and the number of pesticide applications should be reduced as much as possible to protect the natural enemies of aphids and use them to control aphid numbers. Natural enemies can also be artificially reared or captured and released in the field to control aphids.

5. Promptly clean the field, remove old strawberry leaves, and eliminate weeds.

White Grubs

Damage characteristics: White grubs are the general name for the larvae of various scarab beetles. White grubs chew and feed underground on germinating seeds and bite off the roots and stems of seedlings, causing the whole plant to die and, in severe cases, causing missing seedlings and broken rows.

Control methods:

1. Agricultural measures. Arrange crop succession reasonably and implement crop rotation. Farmyard manure used should be fully decomposed to avoid bringing larvae and eggs into the vegetable field or attracting adults to come and lay eggs. Applying chemical fertilizers such as ammonium bicarbonate, ammonium humate, ammonia water, and ammoniated calcium phosphate, the ammonia gas they release has a repellent effect on underground pests such as white grubs. Carry out autumn plowing at the right time to turn some adults and larvae to the surface, so that they dry out, freeze to death, are preyed on by natural enemies, or are mechanically killed.

2. Manual capture and killing. Before applying farmyard manure, screen out the white grubs in it. After planting, if seedlings are found to be damaged, the larvae in the soil can be dug out. Taking advantage of the adults' death-feigning behavior, capture or shake them off the crops on which they rest and kill them.

3. Poison bait trapping and killing. Use 25% parathion or phoxim capsule formulation 150~200 g per mu mixed with bait such as millet 5 kg, or 50% parathion emulsifiable concentrate or 50% phoxim emulsifiable concentrate 50~100 g mixed with bait 3~4 kg, and scatter it in the seed furrow, which can achieve good control effects.

4. Physical methods. In areas with the conditions, black light lamps can be set up to trap and kill adults and reduce the number of white grubs.

5. Biological control. Use tea-colored robber flies, scarab black soil wasps, Beauveria bassiana, etc.

6. Chemical soil treatment. Use 50% phoxim emulsifiable concentrate 200~250 g per mu, add water 10 times and spray it onto 25~30 kg fine soil, mix evenly to make poisonous soil, apply it in strips along the rows, then shallow hoe immediately, or scatter the poisonous soil in the seed furrow or on the ground, then plow it in or mix it into barnyard manure and apply it. Use 2% isofenphos-methyl powder 2~3 kg per mu mixed with fine soil 25~30 kg to make poisonous soil. Use 3% isofenphos-methyl granules, 3% carbofuran granules, 5% phoxim granules, or 5% diazinon granules, 2.5~3 kg per mu to treat the soil.

Common Pests and Diseases of Greenhouse Strawberries and Their Control Measures