A Detailed Guide to the Occurrence and Control of Cowpea Rust Disease
Cowpea (scientific name Vigna unguiculata), commonly known as black-eyed pea, ginger bean, belt bean, hanging bean pod. Cowpea is divided into two types, long cowpea and rice cowpea, and belongs to the legume family. Cowpea is an annual plant of the legume family. The stem has three types: dwarf, semi-climbing and climbing. In the south, climbing types are mainly cultivated, followed by dwarf types. It is an annual twining herbaceous plant of the Fabaceae (butterfly-flower family), with 3 leaflets; the terminal leaflet is rhombic-ovate, 5-13 cm long and 4-7 cm wide, with an acute apex and a nearly rounded or broadly cuneate base, hairless on both sides; the lateral leaflets are obliquely ovate; the stipules are ovate, about 1 cm long, with the attachment point extending downward into a short spur. The calyx is bell-shaped and hairless; the corolla is light purple, about 2 cm long, and the upper part of the style bears light yellow hairs on the inside. The pod is linear, drooping, and can reach up to 40 cm long. The flowering and fruiting period is from June to September.
Cowpea requires high temperatures and has strong heat tolerance; the optimum growth temperature is 20-25°C. In summer, it can still set pods normally at high temperatures above 35°C and does not drop its flowers, but it is not tolerant of frost; under prolonged low temperatures below 10°C, growth is inhibited. Cowpea is a short-day crop, but long cowpea grown as a vegetable mostly belongs to the intermediate photoperiod type and is not very strict about daylight requirements; varieties such as Hongzuiyan and Zhijiang 28-2 can be cultivated in spring, summer and autumn in the south. Cowpea has wide adaptability to soil; as long as the field has good drainage and loose soil, it can be planted. The pods are tender, and sufficient fertilizer and water are required during the pod-setting period. Cowpea is a climbing plant, so when the seedlings grow to more than 30 cm, a trellis about 2 m high should be set up in time; the material is usually reeds, thin bamboo poles, thin wooden sticks, etc. The tips of its branches have a twining and climbing habit and will climb upward on their own.
Geographical distribution of cowpea
It is native to India and Myanmar and is mainly distributed in tropical, subtropical and temperate regions. It is commonly cultivated throughout China. It is widely cultivated in tropical and subtropical regions around the world.
Introduction and control of cowpea rust disease
The disease mainly occurs on leaves, and in severe cases can also damage petioles and seed pods. In the early stage, small light yellow spots appear on the underside of the leaves, gradually turn brown, and rise into small pustule-like bumps, then expand into uredinia; the epidermis ruptures and releases red-brown powder, namely urediniospores. In the later stage, black telia form, causing the leaves to deform and fall early. Sometimes uredinia or telia also form on leaf veins and seed pods. If seed pods are infected, they cannot be eaten. In addition, slightly raised chestnut-brown granular dots can sometimes be seen on both the front and back of the leaves, namely the spermogonia of the pathogen; yellow-white coarse velvety material produced on the underside of the leaves is the aecia.
Cowpea rust disease is caused by Uromyces vignae and Uromyces vignae-sinensis. It is a single-host rust fungus and can produce spermatia, aeciospores, urediniospores, teliospores and basidiospores. It is an obligate parasite and only infects cowpea. In northern China, it mainly overwinters as teliospores on diseased residues. In the following spring, when the daily average temperature is 21--28°C and there are moisture and scattered light conditions, after 3--5 d the teliospores germinate and produce basidiospores, which spread by airflow, produce germ tubes, and invade cowpea leaves to cause damage, while also producing spermatia and aeciospores. After the aeciospores mature, they germinate on cowpea leaves and invade to cause damage, then form uredinia and produce urediniospores. After the urediniospores mature, they spread by airflow and cause repeated infection many times until after autumn, or when conditions are unsuitable in the later growth stage of the plant, telia form and produce teliospores to overwinter. However, in the south, the pathogen mainly overwinters and oversummers as urediniospores. At a daily average temperature of 23°C and a relative humidity of 90%, the incubation period is 8--9 d. When the daily average temperature is stable at 24°C and there are continuous cloudy and rainy conditions, epidemics are likely to occur.
Control methods for cowpea rust disease:
① At the early stage of the disease, begin spraying 15% triadimefon wettable powder at 1000--1500 times dilution, or 40% Fuxing emulsifiable concentrate at 8000 times dilution, or 50% carboxin emulsifiable concentrate at 800 times dilution, or 50% sulfur suspension concentrate at 200 times dilution, or 30% solid lime sulfur at 150 times dilution, or 25% Tilt emulsifiable concentrate at 3000 times dilution, or 65% zineb wettable powder at 500 times dilution, or 50% carbendazim wettable powder at 800--1000 times dilution, or 25% Tilt emulsifiable concentrate at 4000 times dilution plus 15% triadimefon wettable powder at 2000 times dilution, once every 10--15 d, for 2--3 consecutive applications.
② Do not water before rain, and do not grow spring and autumn cowpea in continuous plots.