Occurrence and Control Methods of Diseases and Pests in Rehmannia (Scrophulariaceae)

RehmanniaIt is the fresh or dried tuberous root of the Scrophulariaceae plant Rehmannia. The one that is steamed and then dried is called "prepared rehmannia," commonly known as "shudi," and has the functions of clearing heat and cooling blood, promoting fluid production and moistening dryness, regulating menstruation and nourishing blood, and nourishing yin and tonifying the kidney. Raw rehmannia has a slightly sweet rhizome and has the effects of nourishing yin and clearing heat, cooling blood and stopping bleeding, and treats conditions such as restlessness due to heat disease, skin eruptions and rashes, low fever due to yin deficiency, diabetes-like thirst, hematemesis, epistaxis, hematuria, abnormal uterine bleeding, and sore throat. There are mainly 9 kinds of diseases and pests of rehmannia, and the control measures are as follows.

1. Rehmannia leaf spot disease

It starts in April, and causes severe damage during the rainy period from July to August. It damages the leaves of rehmannia, causing round, irregular yellow-brown spots on the leaf surface with small black dots. Control: In the early stage of the disease, spray Bordeaux mixture at 1:1:150, once about every 13 days, for 3 to 4 consecutive times, or use 60% zineb at 500-600 times dilution, spraying once about every 13 days, for 3 to 4 consecutive times. Burn diseased leaves and do a good job of drainage. After August, cool nights and hot days are very favorable for the occurrence of leaf spot disease. When conditions are suitable, it can become epidemic in 5 to 7 days, causing large areas of leaves to curl, dry up, and die. Foliar spraying with Kubizhi or 58% Ridomil or Bordeaux mixture at 1:1:150 can achieve relatively good control results.

2. Rehmannia wilt disease

Also called root rot, it starts in May and is severe from June to July, damaging the roots and aboveground stems. In the early stage of the disease, the petioles show water-soaked brown spots, the petioles rot, and the aboveground parts wilt and droop. Control: Choose high, dry plots for planting; rotate with gramineous crops, rotating about once every 4 years. Set up drainage ditches. Select disease-free seed roots for propagation, and soak the seeds with 50% carbendazim at 1000 times dilution; in the early stage of the disease, irrigate with 50% Tuzet at 1000-1500 times dilution or 50% carbendazim at 1000 times dilution, once every 7-10 days, for 2 to 3 consecutive times. After July, 58% Ridomil, 80% Dacotec M54, 58% metalaxyl?mancozeb, or 50% carbendazim at 600-800 times dilution can be alternately sprayed on the leaves every 7-10 days for prevention, especially before and after rain when spraying should be done promptly. When scattered diseased plants of wilt disease appear in the field, 50% carbendazim or 50% dexon at 500 times dilution plus a small amount of potassium dihydrogen phosphate can be used to drench the roots of diseased plants and surrounding healthy plants, with 500 g of solution per plant, for 2 to 3 consecutive times, to prevent the spread of the disease.

3. Rehmannia viral disease

Also called mosaic disease, it starts in late April and is severe from May to June. Control: Select virus-free seeds for propagation; propagate from virus-free stem tips; control aphids and choose disease-resistant varieties. After full emergence, use 70% mancozeb or 58% metalaxyl?mancozeb or 50% carbendazim, 50% dexon at 500-800 times dilution plus virus A to carry out one comprehensive root drenching of the seedlings, which can prevent and delay the occurrence of viral disease and wilt disease.

4. Soybean cyst nematode disease

It starts in May and is severe from June to October, damaging the roots. The main damage is to the roots; aboveground plants grow poorly, are short, with yellowing stems and leaves, clustered floral organs, and few or no seeds. When diseased plants are pulled up, the root system is seen to be underdeveloped, with fewer lateral roots, more fine roots, and a marked reduction in root nodules, and white granular objects are attached to the roots, namely the female nematodes-cysts of the pathogenic nematode. Control: ① Agricultural measures: rotation, rotating with cereal crops for 1 to 2 years; increasing phosphorus fertilizer application and timely irrigation. ② Chemical control: use 3% carbofuran granules, 2-4 kg per mu, applied in the sowing furrow, then cover with soil after application, or use 3% isofenphos-methyl granules, 5-6 kg per mu, applied in furrows, then cover with soil after application.

5. Cotton red spider mite

Adult and nymph red spider mites suck sap on the underside of leaves in May, and the damaged areas show small yellow-white spots, until the leaves turn brown and dry up. Spraying 48% Lorsban at 1000-2000 times dilution or 50% phoxim at 800-1000 times dilution plus high-efficiency pyrethroid at 800 times dilution, as well as 20% pyridaben and other high-efficiency, low-toxicity insecticides and acaricides, can control the occurrence of the pest after 2 consecutive sprays.

6. Rehmannia false leopard butterfly

It starts from April to May, and the larvae damage the leaves. Control: Clean the field; in the young larval stage, spray with 90% trichlorfon at 800 times dilution to kill them.

7. Rehmannia yellow spot disease

It is infected by viruses carried by aphids and leafhoppers, causing the leaf surface to become yellow-white nearly round spots, with raised veins, unevenness, and wrinkling. Control: Spray with 80% dichlorvos at 1500-2000 times dilution to kill aphids and other pests.

8. Rehmannia ring spot disease

It mainly damages leaves. The lesions are round or nearly round, and some are semicircular or irregular due to restriction by leaf veins, 2-12 mm in size, initially light brown, and later slightly brown or purple-brown in the center, with concentric rings. Black small dots appear in the diseased parts, namely the pathogen's pycnidia. In severe cases, diseased leaves dry up and die. The pathogen overwinters in the soil with pycnidia along with diseased residues, and produces conidia the following year for primary infection and reinfection. This disease occurs earlier than leaf spot disease, generally starting in early May, entering its peak period in June, and gradually decreasing after mid-July. Control: After harvesting rehmannia, clean the field promptly, and burn or deeply bury diseased residues in a centralized manner. Increase phosphorus and potassium fertilizer application, dredge ditches and drain water after rain to prevent moisture retention, and pay attention to thinning out diseased leaves. Begin spraying 70% mancozeb wettable powder at 500 times dilution or 75% chlorothalonil wettable powder at 600-700 times dilution at the early stage of the disease, once every 7-10 days.

9. Underground pests

The main pests are cutworms, mole crickets, and wireworms. 100 g of 80% trichlorfon wettable powder plus a small amount of water can be mixed with 5 kg of roasted wheat bran or soybean cake and scattered in the evening for trapping and killing.

In the control of diseases and pests of rehmannia, it is strictly prohibited to use pesticides with high toxicity, extreme toxicity, high residues, or "three-causing" effects, so as to avoid pesticide residues polluting the environment and causing harm to human health.