Downy mildew of alfalfa: chlorotic spots appear on the front of leaves, with irregular shape and no obvious edge. There is a gray-white (lavender) downy mildew layer on the back of the leaves, and the periphery is yellow, and the leaves are mostly curled downward. The stems of the affected plants twisted and became thicker, the internodes became shorter and the whole plant turned green. When spores are produced, the lesion becomes grayish brown and is easy to fall off. Alfalfa downy mildew mainly occurs in Chuanyuan irrigation area and humid area, and it occurs earlier, starting from late April to early May, with two peaks in early June and late July, and the disease basically stops developing in late August.

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Alfalfa rust: the damaged leaves first appear faded green spots, and then the spots are slightly convex, spore-like, nearly round or oval. After that, the surface of the spore-like lesion was broken, and brown-red powdery summer spores were scattered, and the leaves shrank, curled and even fell off. In severe cases, the stem can also be infected. Alfalfa rust mainly occurs in arid and semi-arid areas, and it is most popular from mid-late July to mid-early August.

Alfalfa powdery mildew: there is a white mold layer on leaves, stems or pods at the beginning of the disease, and then it gradually expands to produce conidia. In the late stage of the disease, gray or light brown ascospores appear on the coating, with yellow or black spots on it, which makes the plant grow poorly and easy to lose its leaves. Alfalfa powdery mildew occurred from mid-late May to mid-September, and it was the most serious in arid and semi-arid areas, followed by cold and humid areas, and occasionally occurred in Chuanyuan irrigation area.

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Alfalfa brown spot: caused by alfalfa false fungus disk. At the beginning of the disease, there are light brown-green spots on stems, leaves and pods, and then they expand into circles, mostly in the lower stems and leaves, and then spread to the upper tender leaves. In the later stage, brown disc-shaped bulge (sac) appeared in the lesion, and the leaves turned yellow and fell off. Alfalfa brown spot mainly occurs in the first half of June) in the middle of July.

Alfalfa macular disease: light brown spots appear on the damaged leaves at the initial stage, then expand into green spots and then turn into large yellow spots. The lesions were fan-shaped, strip-shaped or round. The diseased leaves dry up and fall off early. There are many orange-yellow to black cup-shaped spots on the tissues of late-stage diseases, that is, the cysts of bacteria, which are mostly born on the back of dead leaves. The onset time is from May to August, which mainly harms young alfalfa and first-season alfalfa, but under the condition of rainy and low temperature, alfalfa in cold and humid areas is the most serious.

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Alfalfa anthracnose: the stems, leaves and pods of plants are infected, and the roots are the most vulnerable, and the plants wither or even die in severe cases. Plants are small and grow poorly, which often causes grassland land to be exposed. The focus is slightly depressed, with brown edges and licorice-colored edges with many black spots on it. When the disease is serious, the lesion can surround the stem or connect with each other, resulting in the stem fracture. The peak period of onset is from early June to early September.

Prevention and control countermeasures

1. Introduce new varieties: alfalfa varieties resistant to pests and diseases can effectively reduce the population density of pests and the infection of pathogenic bacteria, and reduce or avoid the use of pesticides. At the same time, it is necessary to rationally arrange alfalfa varieties to prevent the spread and prevalence of pests and diseases and to avoid planting the same variety in a large area in the same area. When introducing alfalfa seeds, seed quarantine should be strictly controlled to prevent the introduction of dangerous pests and diseases.

2. Pay attention to and strengthen biological control: effectively protect and utilize natural enemy resources. In the control of alfalfa diseases and insect pests, natural enemies should be fully protected and utilized to maintain the ecological balance of alfalfa fields. Alfalfa has obvious advantages in preying on stinkbug insects, ladybugs, Chrysopa and other populations, and has obvious inhibitory effects on aphids, thrips and other pests. Therefore, when taking every control measure, especially chemical control, we should first consider the protection of natural enemies.

3. Timely pesticide control: When the above measures are ineffective in production, biological pesticides with high selectivity and chemicals with high efficiency, low residue and low toxicity can be used for control. You can also use protective trapping method, that is, leave an uncut field on the edge of alfalfa field as bait belt to carry out chemical control when pests migrate to the bait belt. It is forbidden to use highly toxic organophosphorus pesticides, such as dichlorvos, omethoate and parathion. It is necessary to strictly implement the standard of pesticide safety interval, ensure the safety of grassland to the maximum extent and protect the living environment of natural enemies.