What are the control methods for cucumber target leaf spot?
Cucumber target leaf spot is a fungal disease, and the pathogen is Corynespora cassiicola, so it is also called Corynespora leaf spot. It has been reported in various parts of China in the past, but it was not very serious. In recent years, with the continuous development of protected cultivation, the disease has worsened year by year and has become an important leaf disease in protected cucumber production, and may also cause damage to tomatoes and many other vegetables.
Cucumber target leaf spot is a fungal disease, and the pathogen is Corynespora cassiicola, so it is also called Corynespora leaf spot. It has been reported in various parts of China in the past, but it was not very serious. In recent years, with the continuous development of protected cultivation, the disease has worsened year by year and has become an important leaf disease in protected cucumber production, and may also cause damage to tomatoes and many other vegetables.
Cucumber target leaf spot mostly occurs after the peak fruiting stage of cucumber, first on the middle and lower leaves, forming yellow, round, sunken spots on the upper surface of the leaves, with a water-soaked appearance on the underside, commonly known as small yellow spot disease. It gradually expands into brown round or nearly round spots of varying sizes. In the later stage, the color in the middle of the spot becomes lighter, and there is an obvious eye-like target center in the middle of the spot, hence the name target leaf spot. Affected by temperature, humidity, and cucumber disease resistance, the spot size, color, and mold layer vary greatly. Most are 8-15 mm in diameter. When humidity is high, pale grayish-brown moldy growth appears on both the upper and lower surfaces of the spots. When the disease is severe, the spots merge, the leaves wither and turn yellow, and elliptical grayish-brown spots also appear on the stems and petioles.
In the field, cucumber target leaf spot is easily misdiagnosed as downy mildew or bacterial angular leaf spot. When identifying it, pay attention to the following characteristics: the edges of spots of downy mildew and bacterial angular leaf spot are both water-soaked, especially obvious in the morning. In contrast, the boundaries of typical target leaf spot spots are clear; the expansion of downy mildew and bacterial angular leaf spot spots is limited by leaf veins, so they appear polygonal, while target leaf spot spots are slightly round (especially in the early stage of the disease); when humidity is high, downy mildew spots produce a grayish-black mold layer on the underside of the leaf, relatively sparse, while bacterial angular leaf spot exudes milky white turbid water-droplet-like bacterial ooze on the underside of the leaf, leaving white marks after drying, and target leaf spot produces a grayish-brown mold layer on both the upper and lower surfaces of the spots. In addition, leaf spot diseases caused by Alternaria or Pseudocercospora fungi may also occur on cucumber, and since the control techniques are similar, strict differentiation is unnecessary.
Cucumber target leaf spot can be transmitted with seeds, including grafted pumpkin seeds, but more often the pathogen overwinters in the soil as conidia, chlamydospores, or mycelium with diseased plant residues, and produces conidia the following year, which are spread by airflow or raindrop splashing. This disease is an airborne disease of the high-temperature, high-humidity type. Large day-night temperature differences, long dew periods, weak plants, and fields with excessive nitrogen fertilizer application have more severe disease incidence.
To control target leaf spot, the following measures should be taken: soak cucumber seeds and black-seeded pumpkin seeds used for grafting in warm water at 55°C for 30 minutes; rotate with celery, radish, tobacco, bitter melon, or non-cucurbit crops for more than two years, and pay attention to thoroughly removing diseased plant residues in the field and then deep plowing the soil; avoid excessive nitrogen fertilizer, increase phosphorus and potassium fertilizer, and apply an appropriate amount of boron fertilizer. After watering, pay attention to ventilation and dehumidification, and remove diseased leaves in the early stage of the disease; carry out chemical control promptly in the early stage of the disease. The pesticides for controlling target leaf spot are different from those for controlling downy mildew and bacterial angular leaf spot. There are many types to choose from, including broad-spectrum fungicides mancozeb, carbendazim, thiophanate-methyl, and chlorothalonil; triazole agents propiconazole, difenoconazole, flusilazole, and tebuconazole; imidazole fungicides prochloraz and triflumizole; strobilurin agents kresoxim-methyl and pyraclostrobin; dicarboximide agents iprodione; as well as copper quinoline and amistar, etc. Because the pathogen easily develops resistance, the number of applications should be reduced as much as possible, and different types of agents and compound agents should be used in rotation.