Citrus foot rot, also known as skirt rot, is a serious disease that primarily affects the root and lower trunk of citrus trees. It can lead to the decay of main and lateral roots, causing significant damage to the tree's health and productivity. The initial symptoms include brownish water-soaked lesions on the root and neck cortex, accompanied by a fermented smell and the exudation of a sticky substance. When dry, this sticky material hardens into granules. During wet and rainy seasons, the disease spreads rapidly, affecting the bark and even reaching the xylem. In severe cases, the root bark may rot completely, exposing the woody tissue beneath.
When the infection progresses slowly or stops, the surrounding healthy tissue may heal naturally. However, during dry periods, the affected area's cortex shrinks and hardens, leading to cracking or peeling of the bark. Under favorable conditions, the disease can reappear and spread again. As the disease develops, the canopy above the affected root neck often shows signs such as smaller, duller leaves, yellowing, and premature leaf drop, along with branch dieback.
The pathogens responsible for citrus foot rot are mainly species of *Phytophthora*, including *Phytophthora parasitica* and *P. citrophthora*. In some regions like Hunan and Sichuan, *Fusarium* species are also suspected as causative agents.
This disease thrives in warm, humid conditions and is most commonly found in sweet oranges, lemons, and other citrus varieties. Oranges, grapefruits, and Satsuma mandarins are also susceptible, while wolfberry and limes show high resistance. Older trees tend to suffer more from the disease. Factors such as deep planting, poor soil aeration, improper fertilization, pest infestations, low-lying fields, waterlogged soils, and inadequate pruning can all contribute to the occurrence and severity of the disease.
To manage citrus foot rot effectively, several control measures should be implemented:
1. Use disease-resistant rootstocks such as *Cleopatra* mandarin, sour orange, taro orange, or perfume orange. Avoid using citrus as a rootstock in low-lying areas and raise the graft union height if necessary.
2. Plant trees at an appropriate depth, ensuring the root neck remains exposed. Avoid damaging the bark during cultivation, and after heavy rains, ensure proper drainage. Do not apply unripe manure, and manage pests such as root borers.
3. Treat infected trees promptly. Remove the soil around the root neck, then carefully scrape off the diseased tissue along with a 0.5–1 cm margin of healthy tissue. Apply fungicides such as 843 Recovery Agent, 72% Precoce, 70% Pak Dewet, or 25% Metalaxyl wettable powder diluted to 150–200 times. Apply the treatment 1–2 times every 7 days, or up to 3 times if needed. After scraping, fill the wound with sterilized soil mixed with the same fungicide (0.5–1% by weight) or apply 30% soil bacteria solution diluted 500–1000 times, using 3–5 liters per plant.
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