How to identify and diagnose the main pests and diseases of melons and vegetables?
1. Downy mildew: It mainly damages the leaves of cucurbit vegetables, and is most common in cucumber. It is a very contagious common disease. Once it occurs, if conditions are suitable, the disease develops extremely quickly, and a large number of leaves dry up in a short time, commonly known as "running horse dry," directly affecting fruit setting or causing early vine pulling, often resulting in severe yield reduction. The symptoms are mainly on the leaves. At the seedling stage, the cotyledons become chlorotic and gradually show yellow, irregular lesions. When humid, a gray-black mold layer forms on the underside of the leaves, and the cotyledons quickly turn yellow and dry up. On leaves of infected mature plants, water-soaked spots appear, which after expanding are restricted by the leaf veins and become polygonal. In the later stage, the lesions merge into patches, and the whole leaf dries up. Under humid conditions, a light purple to gray-black mold layer forms on the underside of the leaves. The condition most closely related to the occurrence of downy mildew is air humidity. When the relative humidity reaches above 83%, after 4 hours, a large number of sporangia can be produced on the lesions and spread by airflow for infection. In protected cultivation, it can also spread over short distances by water droplets on the greenhouse film.
In addition, applying too much nitrogen fertilizer is also an important cause of downy mildew.
2. Gray mold: Gray mold caused mainly by Botrytis cinerea, and the conidia of this pathogen are mainly spread by airflow. The disease damages the stems, leaves, flowers, and fruits of various vegetables, causing seedling rot and fruit rot. When humid, the diseased part produces a gray-white or gray-brown mold layer, and the surface is densely covered with gray-brown mold. Affected melons in mild cases stop growing and the melon tip rots, while in severe cases the whole melon rots. Leaf lesions are water-soaked, and sometimes a gray-brown mold layer forms in the center of the lesions. Large lesions often appear on leaves, with concentric rings and distinct edges. It can infect and damage tomatoes, cucumbers, hot (sweet) peppers, zucchini, eggplants, kidney beans, and other vegetables to varying degrees, with protected cultivation vegetables being more severely affected. Among them, tomatoes, sweet peppers, and zucchini are the most severely affected, with damage being more serious during the flowering and fruiting stage. The rate of affected fruits reaches about 20%, and in severely diseased greenhouses under protected cultivation, yield loss can reach more than 80%. The stems and nodes and leaves at the seedling and mature plant stages can also be affected, but relatively lightly. In addition to the lack of disease-resistant varieties and continuous rainy weather, the main environmental conditions causing severe gray mold are low temperature (below 25°C for a long time), high humidity (above 83% for a long time), and weak light in protected greenhouses.
3. Phytophthora blight: The most common are tomato late blight and hot (sweet) pepper phytophthora blight. Tomato late blight is a relatively common disease and mainly damages stems, leaves, and fruits. It starts from the seedling stage, develops from leaves to stems, appears dark brown, and rots. When humid, a white mold layer is produced. On leaves of mature plants, water-soaked lesions mostly appear at the leaf tip and leaf margin, gradually turning dark green, and a white mold layer grows on the underside of the lesions. Infection on stems appears as water-soaked lesions that gradually turn dark brown. If green fruits are affected, oil-soaked dark green lesions form near the fruit stalk, gradually turning brown, and the lesions are relatively hard rather than soft. In severely diseased fields, yield loss can reach more than 80%, and in serious cases even total crop failure. After tomato late blight first forms a central diseased plant, it can spread through the entire greenhouse in about 10 days. The occurrence and epidemic of this disease are extremely closely related to climate and environmental conditions. Whenever there are long periods of low temperature, continuous rain, and frequent fog and dew, severe occurrence is possible.
In epidemic years of hot (sweet) pepper phytophthora blight, the general diseased plant rate is about 10%, and in severely diseased fields it exceeds 35%. Because of continuous high temperature and high humidity in protected cultivation and the accumulation of many diseased residues and a large amount of bacteria in soil after 3-4 years of continuous cropping, infection first invades from the stem base of plants at the soil surface when the disease occurs, showing water-soaked lesions, then spreads around the stem base epidermis, presenting dark brown elongated diseased sections, causing all green branches and leaves above the diseased part to wilt rapidly and die. In open-field hot (sweet) pepper phytophthora blight, infection generally first occurs on young tender branches and leaves in the upper part of the plant or at branching points. The underground roots of hot (sweet) pepper phytophthora blight remain normal, which is the main symptom distinguishing it from Fusarium solani root rot.
4. Powdery mildew: Powdery mildew is an important disease of cucurbit vegetables, mainly damaging cucumber, zucchini, melon, pumpkin, winter melon, and others. It mainly infects leaves and also damages stems when severe, but generally does not damage the melon. At the early stage of the disease, small white spots appear on both the upper and lower sides of the leaves, gradually expanding into round white powdery spots. Later, the powdery spots spread outward and connect into patches, becoming large powdery white areas with irregular edges, and can cover the entire leaf. They then turn gray-white, sometimes with many small black dots on them, and the leaves gradually turn yellow and become brittle. Leaves of different ages also differ in resistance to powdery mildew. Tender leaves and old leaves are relatively resistant, while leaves are most susceptible 16-23 days after unfolding. Powdery mildew is more likely to become epidemic when high temperature and dryness alternate with high temperature and high humidity, when there is a large amount of inoculum in protected cultivation, and when susceptible varieties are planted.
5. Anthracnose: Vegetables in the Brassicaceae, Solanaceae, Cucurbitaceae, and Fabaceae families can all be affected, with cucurbit anthracnose and pepper anthracnose being the most common.
When cucurbit seedlings are affected, brown semicircular or circular lesions appear on the edges of the cotyledons. When the stem base is affected, the diseased part constricts and changes color, and the seedlings damp off. At the mature plant stage, leaves, stems and vines, and melons and fruits can all be affected, with slight differences in symptoms among different cucurbits. On cucumber leaves at the mature plant stage, the lesions are nearly circular and vary in size. They are initially water-soaked and quickly dry into red-brown, with a yellow halo at the edge. Small black dots are arranged in concentric rings on the lesions. When humid, pink sticky material forms on the lesions. Under dry conditions, the lesions often crack and perforate. Lesions on stems are white to dark brown, slightly sunken, with small pink dots on the surface. On melons, circular light green sunken lesions appear, and later pink sticky material forms on the lesions. On watermelon leaves, water-soaked spindle-shaped or circular spots first appear and quickly develop into black circular lesions with a purple-black halo outside, sometimes with concentric rings. Under humid conditions, small pink dots appear on the lesions on the upper leaf surface and later turn black. When stems and vines and leaf stalks are affected, the lesions are oblong or elliptical, slightly sunken, first yellow-brown and water-soaked, then turning black. If a lesion extends around the stem vine or leaf stalk, it causes the entire stem vine or entire leaf to die. When watermelon fruits are affected, dark green water-soaked small spots first appear, then expand into circular or elliptical sunken dark brown or black-brown lesions, and the sunken areas crack. Under humid conditions, pink sticky material forms on the lesions. Affected young fruits often turn black, shrivel, and rot.
Pepper anthracnose is divided into black anthracnose, black spot anthracnose, and red anthracnose according to the different infecting pathogens. In black anthracnose, both fruits and leaves can be affected. Fruit lesions are brown, water-soaked, long circular or irregular, sunken, with slightly raised concentric ring patterns, on which numerous small black dots are produced, and there is a moist discoloration ring around the edge. On leaves, the lesions first appear as chlorotic water-soaked spots and gradually turn brown, becoming slightly circular with a gray-white center, on which small black dots are arranged in concentric rings. Brown lesions form on stems and fruit stalks, slightly sunken and irregular, and crack easily when dry. In black spot anthracnose, mature fruits are severely affected, and the lesions closely resemble those of black anthracnose, but the small black dots on the lesions are larger and darker, and under humid conditions sticky material can overflow from the small black dots. In red anthracnose, both mature fruits and young fruits can be affected. The lesions are circular, yellow-brown, water-soaked, and sunken, and small orange-red dots arranged in roughly concentric rings form on the lesions. Under humid conditions, pale red sticky material overflows from the entire surface of the lesions.
6. Cucurbit Fusarium wilt: Cucumber, watermelon, and muskmelon are the most severely affected. The most typical symptom is wilting. At the early stage of the disease, the leaves gradually wilt from bottom to top, resembling a lack of water, more obvious at noon, and can still recover in the morning and evening. After a few days, the leaves of the whole plant wilt and droop and no longer recover to normal. The stem base often constricts, the roots rot brown, and the stem base often splits longitudinally. When the diseased stem is cut longitudinally and examined, its vascular bundles are brown. Under humid conditions, a white or pink mold layer often forms on the surface of the diseased part.
7. Bacterial diseases: Bacterial diseases generally show no visible signs to the naked eye, and the main symptoms include necrosis, rot, and wilting. Bacterial necrosis symptoms are mainly leaf blight and leaf spot, and leaf spots are often water-soaked, such as cucumber bacterial angular leaf spot, bean phytophthora blight, and black rot of Brassicaceae. Bacterial rot has a foul odor, and the rotting tissue often has yellow bacterial ooze, while fungal rot does not, which is the main difference between the rot symptoms of the two types of diseases. Bacterial wilt often causes bacterial wilt with green foliage, while fungal wilt is often accompanied by yellowing.
8. Nematode diseases: Nematode diseases can occur in almost all vegetable types, with root-knot nematode disease being the main one. The disease occurs in the roots, with lateral roots and branch roots being most susceptible, and generally there are no obvious symptoms in the aboveground part. After the roots are affected, root knots or root galls can be produced, and the size of the root knots varies with different vegetable host species and different root-knot nematode species. Severely diseased plants grow weakly in the aboveground part, with yellow and withered leaves and stunted plants.
9. Physiological diseases: Due to the heavy application of pesticides and chemical fertilizers and the special microclimate of protected cultivation, vegetable physiological diseases are becoming more and more serious. There are many types of physiological diseases with varying symptoms, mostly showing compound symptoms, and they are not easy to diagnose. At present, the common physiological diseases mainly include the following: root rot caused by improper watering; blossom-end rot caused by nutrient deficiency; root burning and seedling damage caused by applying undecomposed chicken manure; discoloration of heart leaves or scorched leaf margins caused by excess ammonia in quick-acting chemical fertilizers, as well as leaf curling, leaf margin yellowing, and yellowing of leaf veins caused by excess of other elements; deformed leaves and deformed fruits caused by excessive and abusive use of plant growth regulators; and various types of pesticide damage caused by excessive dosage or too high concentration of pesticides, and so on.