Apple brown spot

Apple brown spot is a fungal disease that primarily affects the leaves, but can also infect fruits and petioles. It causes significant damage to apple trees, especially under favorable environmental conditions. Symptoms of the disease usually begin on the lower parts of the tree canopy and inner leaves. Initially, small brown spots appear, ranging from 0.2 to 0.5 mm in diameter. These lesions start as yellowish-brown spots and gradually expand into round, dark brown lesions with a yellow halo and a green margin. In the center of the lesion, visible dark spots arranged in a rouleau pattern can be seen, which are the conidia of the pathogen. Some lesions may take a lancet-like shape, expanding radially without a fixed form, and may show slight elevation. As the disease progresses, the leaves turn yellow, while the affected areas remain brown and the underside stays green. The lesions are generally smaller than lenticels. The disease cycle begins when the pathogen overwinters in infected fallen leaves on the ground, using hyphae, conidial discs, or apical discs. In spring and summer, conidiospores and ascospores are produced and spread by wind and rain. They typically invade through the leaf surface, with the back of the leaf being the primary entry point. The incubation period ranges from 6 to 12 days, though it can extend up to 45 days in dry years. Higher temperatures shorten the incubation period. From infection to leaf drop, the process takes about 13 to 55 days. The disease typically starts in May and June, peaks between July and August, and declines by October. The disease is more prevalent in winters that are not dry and in springs with early and frequent rains. Years with advanced and heavy spring and autumn rains often lead to widespread outbreaks. Younger trees of the same species tend to be more resistant than older ones. Within the same tree, fruiting branches have a higher incidence compared to non-fruiting branches. Lesions are more common in the lower part of the canopy, where conditions such as shade, poor air circulation, and high humidity favor disease development. To manage apple brown spot, proper orchard management is essential. Improve soil drainage in areas with high clay content or high water tables, and maintain optimal soil moisture. Prune trees regularly to improve air circulation and light penetration, reducing disease pressure. Remove infected leaves in autumn and winter to eliminate overwintering inoculum. Chemical control options include applying Bordeaux mixture (1:2:200), 30% mancozeb suspension (300–500 times), 36% thiophanate-methyl (500 times), 70% mancozeb WP (500–600 times), 50% wettable sulfur (500–600 times), 50% acetamiprid wettable powder (1000–1500 times), 10% propiconazole WP (1000–1500 times), 64% azoxystrobin WP (500–600 times), 80% antibacterial agent 402 EC (800–1000 times), 60% carbendazim ultrafine powder (600–800 times), and 50% thiophanate-sulfur suspension (800 times). Spraying should begin after flowering, combined with powdery mildew or other foliar diseases, and repeated every 20 days for 3–4 applications. Adding 0.5–1% soy milk, "6501" adhesive (1000 times), or leather (3000–4000 times) can enhance spray retention and effectiveness.

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