What Are the Symptoms of Walnut Anthracnose? A Detailed Guide to Prevention and Control Methods
1. Distribution and occurrence characteristics Walnut anthracnose generally occurs mostly from June to August, with slight variations from place to place. In Jiangsu, Henan and Shandong it occurs from late June to early July, and in Hebei and Liaoning in August. The pathogen overwinters as mycelium and conidia in diseased fruits, diseased leaves or bud scales, and the following year produces conidia that are spread by wind and rain or insects, invading through wounds or natural openings; after infection it produces spore masses that are spread by rain splash, causing multiple secondary infections. The timing and severity of the disease are closely related to the rainfall of that year; generally, frequent rainy days, high humidity and poor ventilation and light penetration favour the disease. If the rainy season comes early and there is plenty of rain that year, the disease occurs early and severely; otherwise it occurs late or mildly. Disease resistance differs among varieties: the high-yielding thin-shelled types of Aksu and Kuqa in Xinjiang are susceptible, while late-maturing varieties are only mildly affected.
2. Pathogen and damage symptoms This is a fungal infection disease. Its sexual stage is called Glomerella, a fungus of the phylum Ascomycota. Its asexual stage is Colletotrichum gloeosporioides, a fungus of the class Deuteromycetes.
It mainly damages walnut fruits, and also occurs on leaves, buds and young shoots. The usual diseased-fruit rate is 20%-40%. In severe cases it can reach more than 90%, causing the walnut kernels to shrivel and greatly reducing both yield and quality. It causes fairly serious damage to walnuts in various places.
On the fruit, the spots are initially brown, later turning black, nearly round, with a sunken centre. On the spots are many raised brown to black dots, sometimes arranged in concentric rings. When humidity is high, the small black dots on the spots appear as small pink raised bumps, which are the pathogen's acervuli. A single diseased fruit has from one to a dozen spots; the spots may enlarge or merge, which can cause the whole fruit to blacken and rot.
Leaf spots occur less often; they are nearly round or irregular, some extending along the leaf margin, and some extending in long strips along both sides of the midrib and lateral veins. In severe cases the whole leaf turns yellow and withers. When humidity is high, the small black dots on the spots appear as small pink raised bumps, which are the pathogen's acervuli and conidia.
3. Prevention and control methods
(1) In winter, remove diseased fruits and diseased leaves and burn or bury them deeply to reduce sources of infection; in June and July, promptly pick off diseased fruits.
(2) When planting, the spacing between plants and rows should not be too dense, so as to ensure good ventilation and light penetration.
(3) Chemical control. Before bud break, use 3-5 degrees Baume lime sulfur mixture; after flowering and before the disease appears, use 1:1:200 Bordeaux mixture or 50% Tuzet (thiram) 600-1,000 times dilution; the young fruit stage is the key period for control. During the disease period, you can also use Duofu mancozeb 1,000-1,500 times dilution + 0.03% skin glue, 2% Agricultural Antibiotic 120 water-based formulation 200 times dilution, 50% thiophanate-methyl 800-1,000 times dilution, or 50% carbendazim wettable powder 800-1,000 times dilution.