ROOT ROT YELLOW LEAF  DISEASE ON CITRUS TREES 26/05/2022

In recent years, the area of citrus trees in the Mekong River Delta has increased significantly, due to the high profits of intensive growers. However, the increase in acreage and intensification causes growers to face many challenges.

One of the top challenges is the complication of the pest situation. Gardeners have to live with dangerous pests such as yellow leaf disease, green veins, ulcers, scabs, red spiders, fruit borers, yellow leaf rot, root rot... In which  root rot yellow leaf is the most ominous pest.

Root rot yellow leaf is caused by a variety of pathogens including Fusarium, Pythium, Phytophthora, Rhizoctonia, nematodes and root mites. The disease does not appear immediately in the rainy season, when the soil is soggy, but the disease often appears seriously in the early sunny season.

When the disease first appeared the leaves of the tree were still normal but the veins were pale yellow, the leaf blades turned orange-yellow and fell easily. When the wind blew, the lower leaves fell first, then the upper leaves; Fruit quality is poor and falls prematurely; Severe disease can kill the whole tree, if not treated in time.

In which direction the branch is diseased, the roots will often rot in that direction. Root system rot from small roots spread gradually into large roots. Rotten roots are brown, root bark slips out of the wood, inside there are brown stripes that gradually spread to the taproot. The roots lose the ability to absorb water and nutrients to nourish the plant, thereby causing the branches to die. When seriously diseased, all the roots turn black and die, eventually the whole plant dies.

Causes and conditions of disease occurrence:

Direct cause:

- Nematodes, soil mealybugs and root-damaging spiders are the main causative agents of root rot yellow leaf disease. Because they create wounds on the roots, reduce the immunity of the roots, thereby creating favorable conditions for the fungal combination (Fusarium, Pythium, Phytophthora, Rhizoctonia) to invade and cause harm.

Indirect causes:

- The garden is made with low beds and mounds, poor drainage and often  being flooded, or in the process of  reducing water of flowering treatment  to weaken the roots, some roots go deep down when the rain comes, the untimely drainage  makes the roots waterlogged, lack of oxygen leads to roten root tips and  suction hairs.

- The garden uses less organic fertilizers and overuses chemical fertilizers. Especially using fertilizers containing NAA, IAA growth stimulants..., excessively leading to rapid aging of the roots and breaking the soil structure, causing beneficial antagonistic microorganisms in the soil to be destroyed and the harmful microorganisms reproduce quickly. From there, weakening the roots and weakening the immune system of the roots, the young roots gradually die, creating opportunities for fungal combinations (Fusarium, Pythium, Phytophthora, Rhizoctonia) to develop.

Measures to prevent root rot yellow leaf on citrus trees:

- The garden for growing citrus trees must be high and dry and well-drained. If the garden is low, the embankment must be built to control water in the rainy season.

- Select disease-free seedlings. The garden must have a good drainage system.

- Prune branches, shape the tree as soon as the tree is young, regularly prune, remove old weak branches, pests.

- Remove severely diseased plants that are not able to recover,

- Apply lime at the beginning of the rainy season to disinfect, prevent fungal diseases and raise soil pH to help plants grow well.

- Check the garden regularly to detect the earliest for timely preventive measures.

- Increase the use of organic fertilizers, especially microbial organic fertilizers (eg HCVS1 VIETSTAR fertilizer) to stabilize the soil structure, increase humus, loose soil and destroy harmful pathogens present in the soil.

- Should add CALCIUM NITRATE 3 times a year to increase the immunity of the roots and the resistance of the plants.

- Treatment of nematodes, soil mealybugs and root-damaging spiders around the root zone such as: Comda 250EC, Sago Super 20EC, periodically 2-3 times/year. Especially in the dry season.

- Root rot yellow leaf disease is very difficult to treat, so it is necessary to pay attention to monitor the garden and spray early to prevent disease.

Recently, Saigon Plant Protection Joint Stock Company has conducted a number of tests in Lai Vung - Dong Thap and has selected 2 spray treatments  with good effect on preventing root rot yellow leaf disease on citrus trees:

• Treatment 1: Mix 500g Pylacol 700WP + 480ml Treppach Bul 607SL for 200 liters of water, irrigate 250m2, water evenly on the bed surface

• Treatment 2: Mix 500gr Pylacol 700WP + 500gr Mexyl MZ 72WP for 200 liters of water, water for 250m2, evenly water on the bed surface

Preventive spraying: Treat the garden with treatment 1 or treatment 2, watering 3 times a year at the beginning of the rainy season, in the middle of the rainy season and at the end of the rainy season. Combine spraying Dipomate 430SC through leaves after finishing each watering for 03 days, with a dose of 1.0 liter Dipomate 430SC for 200- liter-water drum to spray evenly on foliage.

 Spray when early disease appears scattered on a few branches on the canopy: Treat immediately with treament 1 or treament 2, watering 03 times, each time 07 days apart. Combine spraying Dipomate 430SC through the leaves after finishing the last watering for 3 days, with a dosage of 1.0 liter of Dipomate 430SC for a drum of 200-liter-waterdrum to spray evenly on the canopy.