The false black spot disease of rapeseed, also known as the small black spot, primarily affects the leaves of the plant. In severe cases, the lesions merge and cause the leaves to wither and die. This disease often co-occurs with black spot, making it even more damaging. Timely application of pesticides is essential to manage this issue. According to the **China Pesticide Network**, Xiaobian has gathered detailed strategies for effective prevention and control.
The causal agent of this disease is *Alternaria alternata* (Fries) Keissl., which was previously known as *A. tenuis* Nees. It is a fungal pathogen that produces conidiophores in clusters—dark brown, short, thick, or irregularly shaped, with 2–9 septa. The conidia are inverted bar-shaped or cylindrical, pale yellow, ranging from 9.5–40 × 5–11.25 micrometers in size, with 1–6 median septa. Most commonly, they have 3–4, but sometimes up to 5. These spores have a short beak, about half the length of the spore, and may contain one or two septa. The vegetative hyphae are typically mononuclear, though some may be binucleate or trinucleate. The tip cells of new hyphae often contain multiple nuclei. Conidia cells are mostly mononuclear, but the widest spores may have 2–3 nuclei. The mycelium forms collaterals, and when forming spore stalks, the daughter cells grow first, with one nucleus migrating from the mother cell. New cells continue dividing into collaterals, conidiophores, and conidia through this nucleus. When conidia germinate, germ tubes emerge from the cells, with only one nucleus per cell moving into the tube. Germination tubes may contain a single or dual nucleus.
Historically, this organism was considered saprophytic, but under favorable conditions, it can become weakly parasitic, causing damage to the host. Thus, what was once a saprophyte can turn into a pathogen.
The disease spreads through mycelium or conidia present on seedlings, seeds, infected plant parts, or in the soil. It overwinters or oversummers in the soil, serving as the initial infection source. Conidia are spread by wind, leading to secondary infections and continuous disease outbreaks. According to the **pesticide network**, the pathogen thrives between 10–35°C, but prefers lower temperatures, with an optimal temperature of 17°C and a pH of 6.6. It can survive in water for up to one month, in soil for three months, and on the soil surface for up to one year. The disease typically occurs during the mid-to-late growth stages of rapeseed or in off-season cultivation, making it prone to outbreak. In recent years, its impact has increased significantly, becoming a major concern in rapeseed production.
Control measures include large-scale crop rotation, plowing fields after harvest to reduce bacterial sources, and maintaining clean fields. Applying compost made with Japanese enzyme bacteria or manure compost can help cultivate beneficial microbes, improving the plant's resistance. Selecting resistant varieties such as Qinbai No. 2, Yuyou No. 3, Zhengbai No. 4, Xiaoyeqing, and Rongbai No. 4 is also recommended. Seed disinfection using 0.4% chlorhexidine WP at 50% of the seed weight, or 75% chlorothalonil WP, can prevent early infection.
For foliar treatments, spraying solutions like 64% antivirus WP diluted 500 times, 40% chlorothalonil suspension 500 times, 70% mancozeb WP 500 times, or 58% metalaxyl MnZn wettable powder 500 times is effective. Other options include 80% spray WP 600 times, 50% phlegm hygroscopic powder, or a 1000-times dilution of its compound. When black spot and downy mildew occur together, mixtures such as 70% ethyl phosphine•manganese zinc wettable powder 500 times, 60% succinylphosphine aluminum (DTM) wettable powder 500 times, 72% frost urea manganese zinc (Kai Anti-Linguistic) wettable powder 800 times, or 69% Anke manganese zinc wettable powder 1000 times are advised. Apply approximately 660 liters of solution per 667 m², repeating every 7–10 days for 3–4 applications. Stop applying **pesticides** seven days before harvesting.
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