Identification and diagnosis of main diseases and pests in melons and vegetables

1, downy mildew: mainly harmful to the leaves of melon vegetables, cucumber is the most common occurrence, is a very popular common disease, once the disease, if the conditions are appropriate, the disease develops very fast, a large number of leaves dry in a short time, commonly known as "horse dry", directly affect the melon or early pull seedlings, often resulting in serious yield reduction. The symptoms are mainly manifested on the leaves. The cotyledons in the seedling stage appear chlorotic, gradually showing yellow irregular disease spots. When wet, the leaf surface produces gray-black mold layer, and the cotyledons quickly turn yellow and dry. In the adult stage, water-soaked spots appear on the infected leaves, which are restricted by the veins after expansion, showing polygonal shapes. In the later stage, the disease spots merge into pieces, the whole leaves dry up, and the back of the leaves produces pale purple to gray-black mold layer under wet conditions. The most important condition for downy mildew is air humidity. When the relative humidity reached 83%, after 4 hours, a large number of sporangia could be produced on the diseased spots, and the infection could be transmitted by air flow. In addition, excessive application of nitrogen fertilizer is also an important cause of downy mildew. 2. Gray mold: Gray mold caused by Botrytis cinerea is the main one, and the conidia of this pathogen are mainly transmitted by air flow. The disease harms the stems, leaves, flowers and fruits of various vegetables, causing rotten seedlings and rotten fruits. When wet, the diseased parts produce gray or brown mold layers, and the surface is densely covered with gray brown mold. The growth of the injured melon is stagnant, the melon head is rotten, and the whole melon is rotten. Leafy spot for water-soaked shape, spots sometimes produced in the middle of gray brown mold layer. There are often large disease spots on the leaves, and there are ring patterns, and the edges are obvious. Different degrees of infection and damage occurred on tomato, cucumber, hot pepper, zucchini, eggplant, kidney bean and other vegetables, so as to protect the vegetables. Tomato, sweet pepper and zucchini were the most seriously affected, especially in flowering and fruiting stage, the rate of damaged fruit was about 20%, and the yield loss of seriously ill greenhouse in protected field could reach more than 80%. Stems and leaves at seedling stage and adult stage can also be infected, but relatively light. Besides the lack of resistant varieties and continuous rainy weather, low temperature (below 25℃ for a long time), high humidity (above 83% for a long time) and weak light were the main environmental conditions causing the serious occurrence of gray mold. 3. Phytophthora blight: The most common ones are tomato late blight and spicy (sweet) pepper blight. Tomato late blight is a common disease, mainly damaging stems, leaves and fruits. Disease starts from seedlings, develops from leaves to stems, is blackish brown, decays, and produces white mold layers when wet. At the mature stage, water-soaked spots appeared on the leaves from the tip and edge of the leaves, gradually dark green, and white mold layer grew on the back of the spots; the infected stems were water-soaked spots, gradually dark brown; at the green fruit stage, the green spots near the fruit stalk formed, gradually brown, and the spots were hard and soft. The yield loss of seriously ill fields can reach more than 80%, and even no yield in serious cases. Tomato late blight can spread to the whole shed in about 10 days after it first forms the central diseased plant. The occurrence and prevalence of this disease is closely related to climate and environmental conditions. Whenever it encounters long-term low temperature, continuous rain, foggy dew and other conditions, it may occur seriously. In epidemic years, the incidence of pepper blight was about 10% in general and over 35% in serious disease fields. Because of the continuous high temperature and high humidity in the protected land, there are many diseased bodies and large bacteria content in the soil for 3-4 years, so the disease first invades from the stem base of the plant on the ground, presents water-soaked lesions, and then expands around the stem base epidermis, presenting black-brown long disease segments, resulting in all green branches and leaves above the diseased part rapidly wilting and dying. Spicy (sweet) pepper blight in the open field is generally first infected in the upper tender branches or branches of the plant. The underground root of pepper blight is normal, which is the main distinguishing symptom from Fusarium rot. 4. Powdery mildew: Powdery mildew is an important disease of melon vegetables. It mainly harms cucumber, zucchini, melon, pumpkin, winter melon, etc. It mainly infects leaves, and also harms stems when it reoccurs. Generally, it does not harm melon. At the beginning of the disease, white spots appear on both sides of the leaves, gradually expanding into round white powdery spots, and then the powdery spots spread around, connecting into pieces, becoming large white powder areas with irregular edges, and can be covered with the whole leaves, and later become gray white, sometimes there are many small black spots on them, and the leaves gradually become yellow and brittle. Young leaves and old leaves were more resistant to powdery mildew and were most susceptible to powdery mildew 16-23 days after leaf expansion. Powdery mildew was more prevalent under the condition of alternating high temperature and high humidity, existence of a large number of bacteria sources in protected areas and planting susceptible varieties. 5. Anthracnose: Cruciferae, Solanaceae, Cucurbitaceae and leguminous vegetables can all be affected, with melon anthracnose and pepper anthracnose being the most common. Cucurbit seedlings disease, cotyledon edge brown semicircular or circular spots; stem base damage, disease contraction, discoloration, seedling damping-off. Adult leaves, stems and vines and melons can be affected, different melons have slightly different symptoms. Cucumber adult stage leaf spot nearly round, size varies, initially water-stained, quickly dried into reddish-brown, yellow halo at the edge, small black spots on the disease, wet spots on the pink sticky material, dry conditions often crack, perforation. Stems white to dark brown spots, slightly concave, pink spots on the surface, melon disease produced round light green concave spots, late spots produced pink goo. Watermelon leaves damaged initially appear water-like spindle or round spots, soon developed into black round spots, the periphery has purple-black halo, sometimes concentric ring. Under wet conditions, pink spots appear on the front of the leaf, and then turn black. Stems and petioles are damaged, the disease spots are oblong or elliptical, slightly concave, first yellow-brown water stains, then black, if the disease spots develop around the stems or petioles for a week, it will cause the whole stems or leaves to die. Watermelon fruit damaged, first dark green water-like small spots, then expanded into round or oval concave dark brown or dark brown spots, concave cracks, wet conditions on the spots produce pink sticky matter. Young fruits often become black, shriveled and rotten when damaged. Pepper anthracnose can be divided into black anthracnose, black spot anthracnose and red anthracnose according to their infection sources. Black anthracnose, fruit and leaves can be affected, fruit spots for brown water-like oval or irregular shape, depressed spots, slightly raised concentric ring pattern spots, which give birth to numerous black dots, the periphery of the wet color circle. The disease spots on the leaves were initially pale green water-stained spots, gradually turning brown, slightly round spots with gray centers and black spots in rotation above. Brown spots on stems and fruiting pedicels, slightly sunken, irregularly shaped, easily dehiscent when dry. Black spot anthracnose, mature fruit seriously affected, the disease spots are very similar to black anthracnose, but the small black spots on the disease spots are larger and darker, and the small black spots can overflow sticky substances under wet conditions. Red anthracnose, mature fruit and young fruit can be affected, round spots, yellow brown, water-stained, concave, orange spots slightly concentric ring arrangement of dots, wet conditions, the whole surface of the spots overflow reddish sticky matter. 6. Melon blight: cucumber, watermelon and cantaloupe were the most seriously affected. The most typical symptom is wilting. At the beginning of the disease, the leaves gradually wilted from bottom to top, like water shortage, more obvious at noon, and can recover in the morning and evening. After a few days, the whole leaves withered and drooped, no longer returning to normal. The stem base often contracted, the root was brown and rotted, the stem base was often split longitudinally. The diseased stem was examined longitudinally, and its vascular bundle was brown. Under wet conditions, white or pink mold layers are often produced on the surface of the diseased part. 7. Bacterial diseases: Bacterial diseases generally do not show symptoms to the naked eye, mainly including necrosis, decay, wilting and other symptoms. Bacterial necrosis symptoms are mainly leaf blight and leaf spots, leaf spots are often water-stained, such as cucumber bacterial angular spot disease, bean blight, cruciferous black rot and so on. Bacterial decay has a foul smell and yellow pus is common in decaying tissues, while fungal decay does not. This is the main difference between the two types of decay symptoms. Bacterial wilt often causes blight, while fungal wilt is often accompanied by yellowing. 8. Nematode disease: Nematode disease can occur in almost all kinds of vegetables, mainly root knot nematode disease. Root disease, lateral roots and branch roots are the most vulnerable, generally there is no obvious symptom on the ground. Root knot or nodule can be produced after root damage, and the size of root knot varies with different vegetable host species and different root knot nematode species. The overground part of the seriously ill plant grows weak, the leaves are withered, and the plant is short. 9. Physiological diseases: Due to the large amount of pesticides, chemical fertilizers and special microclimate cultivated in protected areas, the physiological diseases of vegetables are getting heavier and heavier. There are many kinds of physiological diseases with different symptoms, most of which are complex symptoms and difficult to diagnose. At present, the common physiological diseases mainly include the following: root rot caused by improper watering; umbilical rot caused by lack of nutrients; root burn and seedling injury caused by application of undecomposed chicken manure; fading of heart leaves or scorched leaf edges caused by excessive ammonia in quick-acting chemical fertilizers, leaf curling, leaf edge yellowing, vein yellowing caused by excessive use of other elements; abnormal leaves and abnormal fruits caused by excessive abuse of plant growth regulators; multiple phytotoxicity caused by excessive pesticide dosage or concentration, etc.