Symptoms and Control Methods of Primula Leaf Spot Disease

Symptoms: It mainly affects the leaves of the plant. The disease spreads from the lower part of the plant to the upper part. The spots are usually 3 to 4 mm in diameter and brown; in severe cases, the diseased leaves wither and die, causing defoliation (see Figure 2). The disease is caused by Ramularia primulae. The pathogen overwinters as mycelium or conidiomata on diseased plant debris, and seeds can also carry the pathogen, becoming the primary infection source for the following year. The disease mainly spreads through conidia carried by air and rainwater. Reinfection is frequent during the growing season. Plants are usually prone to the disease in warm, humid weather and when nitrogen fertilizer is applied excessively. The disease generally begins in July and becomes epidemic from August to October.

Control methods: Select and breed disease-resistant varieties, strengthen fertilizer and water management, apply more organic fertilizer and phosphorus-potassium fertilizer, and avoid excessive nitrogen fertilizer. In the early stage of the disease, spray 70% thiophanate-methyl 1000-fold solution plus 75% chlorothalonil wettable powder 1000-fold solution, or 1:1:100 Bordeaux mixture.