A pest management strategic plan for cowpeas does not currently exist at the Regional IPM Center websites. A ranked list of the major pests and research/extension priorities for Georgia generated at the cowpea working group meeting at Savannah, Georgia in January 2017 is provided below.
Major Pests of Cowpeas in the SE USA:
Insects. The insects listed here are ranked from the most important and common pests in Georgia with the emphasis on southern Georgia where the majority of the production occurs. All insect pests, but the cowpea curculio, have satisfactory control options available to growers. The cowpea curculio is the main production-limiting key pest where it occurs in the southeastern USA.
- Cowpea curculio, Chalcodermus aeneus (Coleoptera), is a weevil (see image), that seems to have originated from the Caribbean / Central America region of the New World. It has never been reported in the Old World, but has been reported as the major pest of southern peas where it occurs in the SE USA for well over a century. The distribution of the weevil in the SE USA has been reported roughly in the triangle from southern Texas to North Carolina and south to Florida. However, with the tremendous decline in southern pea acreage over the last 50 years, the distribution is more scattered and tends to be reported more in traditional southern pea production areas of Alabama, Georgia and South Carolina in recent years. Both the larval and adult feeding causes damage to the pea and can make it unmarketable. Feeding and egg laying occurs in the developing pods producing a distinct, dark spot lesion or “sting” on the outside of the pod. Heavy feeding by adults can reduce the amount of flowering and therefore fruit set in the crop. The grub develops inside of the pod feeding directly on the seeds, producing frass (insect excrement) inside of the pod. As much as 40-60% yield loss can be typical.
- Stink bugs, specifically southern green stinkbug, Nezara viridula, and brown stinkbug, Euschistus servus (Hemiptera), are common pest of cowpeas, feeding mainly on the pods during seed development. The external damage appears as a small lesion or “sting”, smaller than that caused by the curculio, and the damage internally is reduced seed weights, but no frass will be present. Stink bugs are highly seasonal and only cause significant damage when they occur in high numbers for short periods during the spring and summer. Also, stinkbugs are relatively easy to control with insecticides which can be timed to scouting reports, eliminating any need for calendar sprays. Thus, the impact of control on pollinators can be much less than for curculio spray programs. Southern green stinkbugs reach damaging levels at 4 stinkbugs foot of row of southern peas in Georgia.
- Armyworms, Spodoptera spp. (Lepidoptera), and in particular, beet armyworm, S. exigua, can cause noticeable damage to the foliage of cowpeas generally during the summer. It has not been documented whether or not armyworm damage actually results in significant yield loss because the cowpea plant tends to compensate for some foliar damage, but the assumption is that 15% foliage loss from two weeks prior to flowering and until pods have filled is a treatment threshold, similar to what is recommended for soybean.
- Cowpea weevil, Callosobruchus maculatus (Coleptera), is a stored grain pest of cowpea that only affects the dried cowpea seed in Georgia, not the fresh frozen product or any field production. Storing dried seed at near freezing temperatures can eliminate the weevil in the seed bags.
- Other insect pests to look out for: Other insects that can cause damage to the plant, but generally occur in low levels are the banded cucumber beetle, Diabrotica balteata (Coleoptera) which is a defoliating pest, kudzu bug, Megacopta cribraria (Hemiptera) which can feed on stems and pods like stinkbugs, Cowpea aphid, Aphis craccivora (Hemiptera) which can transmit mosaic viruses and rarely build up to cause damage on their own without the virus, leafhoppers, Empoasca spp. (Hemiptera) a sporatic pest, corn earworm, Helicoverpa zea (Lepidoptera) which rarely causes problems in cowpeas in Georgia, American serpentine leafminer, Liriomyza trifolii (Diptera), which causes mining in the leaves, but rarely warrents control in Georgia. And the chrysomelid beetle, Cerotoma ruficornis (Coleoptera) which is sporatic.
Disease Agents. The cowpea plant pathogens listed here are ranked from the most important and common in Georgia with the emphasis on southern Georgia where the majority of the production occurs. Most pathogens currently have good chemical control options in Georgia cowpeas.
- Cercospora Leaf Spot (Mycosphaerella cruenta) Causes circular leaf spots and generally not restricted by veins. Lesions often have light brown to gray centers with a reddish border. In time chlorosis of the entire leaf occurs and blighted areas coalesce to become necrotic. Primary source of inoculum can be from crop debris or susceptible legumes in the region as spores are airborne. Although there are effective fungicides available, the low profit margin of cowpeas makes fungicide use an unattractive option in Georgia.
- Choanephora pod rot (Choanephora cucurbitarum) This disease generally follows cowpea curculio or other physical damage on the pods. Initial symptoms are darkened, water-soaked lesions on the pots. In time developing seed and the entire pod succumb to a rather wet, slimy rot. Fungal hyphae will develop and produce dark sporangia and sporangiospores giving the infected area a “fuzzy” appearance. The disease is similar to Choanephora rot of squash and other cucurbits. Damage by cowpea curculio predispose the pods for Choanephora infection. Hence, curculio management helps in managing this pathogen.
- Cowpea mosaic virus can be transmitted mechanically, by aphids and it can be seed-borne. The virus has a wide host range and infects many members of the Chenopodiaceae and Leguminosae. Symptom expression can vary depending on the host infected. In cowpea symptoms are typical of those caused by mosaic viruses, namely chlorotic spots with rings. When severe, leaf distortion, necrosis and plant collapse can occur. On some varieties develop necrotic local lesions. No wide spread outbreaks of virus have been observed in recent years, so no current control actions have been needed.
- Bacterial blight & canker of cowpea (Xanthomonas axonopodis pv. vignicola) symptoms range from angular, vein restricted lesions to large wedge- or pie-shaped blighted areas extending to the leaf margin. Often lesions have a chlorotic halo. Infections produce abundant levels of ethylene, which leads to leaf abscission and defoliation. In Georgia the cream and crowder types are more prone to developing stem cankers and plant lodging. The primary source of inoculum is contaminated seed. This was of major importance when Georgia growers produced their own seed as regional environmental conditions favored seed-borne development.
- Southern blight (Sclerotium rolfsii) is a soil-borne fungus that has an extremely wide host range (> 500 plant species). The fungus is favored by warm, humid or wet conditions. Infections generally occur in lower stems near the soil surface. Soft rot symptoms develop and the fungus girdles the stem. Infected plants wilt, lodge and eventually die. Characteristic signs of infection include a fluffy, white mycelial mat and presence of mustard-seed like sclerotia clustering on infected tissues.
- Rhizoctonia stem canker/damp off is another soil-borne fungus favored by warm wet conditions. In Georgia, Rhizoctonia is generally associated with damping-off of young seedlings. In older plants, reddish-brown stem cankers can appear and can result in plant lodging.
- Other diseases to look out for: Root Knot, Meloidogyne spp. nematodes, Cowpea severe mosaic virus, Anthracnose, Colletotrichum, Fusarium wilt Fusarium oxysporum, Cowpea chlorotic mottle virus, Pythium stem rot, Pythium spp., Pod rot, Botrytis spp., Septoria leaf spot, Septoria spp., Rust, Uromyces appendiculatus, Powdery mildew, Ersyphe polygoni, and Target spot, Corynespora cassicola.
Weeds. Weeds are a major pest group of any vegetable crops and cowpeas or southern peas grown as a vegetable crops are no exceptions. Since cowpeas are typically grown in bare-ground production systems and during the warmest part of the year, similar to snap beans, the types of weeds affecting cowpeas is similar to the weed complex in other summer legume crops in the Southeastern USA, such as morning glory, pigweed, nutsedge, sicklepod and others. Growers usually cultivate southern peas until the plants become too large to pass easily through the cultivator. Later in the growing season if weed control is still needed, herbicides become the main tool for weed management. Weed management of grasses and broadleaf weed species are common, but yellow nutsedge and larger-seeded broad leaves might be needed. The months of cowpea production tend to be between April and October, so winter weeds are almost never an issue.
1) Germplasm development for resistance to curculio and other pests
a) Higgins' alpha amylase inhibitor germplasm
b) Core collection (field testing for resistance, -200 lines for initial screening)
c) High through-put phenotyping
2) Overwintering curculio control program
a) Biological control
b) Basic biology, diapause, movement, etc.
3) Regional distribution and risk of curculio or other pest infestation
a) Identify alternate reproductive hosts for curculio
b) Regional curculio biotypes and host associations
c) Identification of toxicological resistance to insecticides (LCSO's, etc.)
d) Improved trapping with pheromone baits for curculio
e) Ranking of multiple pests of cowpea by location
4) In-season control of curculios with effective insecticides when available
5) Genome editing for novel curculio resistance from cowpea and relatives
6) Weed management
a) herbicide resistant weeds
b) Emerging weeds
7) Disease management
a) Emerging pathogens
8) Harvesting and post-harvest issues
1) Post crop profiles for GA, SC and TN
2) Increasing public awareness of southern pea, crop promotion
3) 24c for dual, 3-day PHI paraquat