Disease Pest Management in Groundnut

Several diseases may attack groundnuts throughout the growing cycle. The most common diseases in Malawi include early leaf spot and late leaf spot, rust, and groundnut rosette disease.

Cultural practices can help to prevent diseases. When farmers also plant resistant varieties, they have the greatest control of diseases and virus, and the best potential for high yield. For example, early planting can reduce aphids which in turn reduce transmission and expression of rosette virus.

Leaf Spot Disease

Early leaf spot is a fungal disease that attacks the groundnut crop soon after emergence. The disease is characterized by lesions (spots) produced on all above-ground parts of the plant (leaves, petioles, stems, and pegs). The lesions are sub-circular and dark brown on the upper surface and light brown on the lower surface of the leaves. In severe cases, the disease causes premature defoliation and has the potential to cause yield loss of up to 50%. This disease is widespread in all groundnut growing areas and is particularly serious in the central region (Lilongwe–Mchinji to Kasungu plain).

Farmers can manage the disease by removing or burying infected crop residues.

Late leaf spot is a fungal disease that occurs late in the season. It is characterized by lesions that are nearly circular and black. In severe cases, affected leaves become chlorotic, then necrotic and lesions coalesce, resulting in premature defoliation. It can cause yield losses ranging from 25% to 50%. This disease occurs sporadically in several parts of Malawi but is widespread in the warm and humid low altitude areas of the Kasungu, Mzuzu, Salima and the Shire Valley ADDs.

Both early and late leaf spot can cause rapid defoliation of the groundnut canopy. Once lesions appear near the base of the plant, compete defoliation can occur within a few weeks. Leaf spot epidemics expand rapidly, as shown in Figure 12. From just a few lesions in the bottom of the canopy, the disease spreads and lesions appear throughout the canopy. Ultimately, significant defoliation is noted 14 days later and then almost complete defoliation in another 7 days. In this example, only 21 days separated the time when lesions first appeared at the base of the plant to almost complete defoliation. Using a leaf spot-resistant variety or an effective fungicide regime can offer protection from leaf spot.

Trials conducted in Malawi in 2020 and 2021 showed a positive effect of fungicide applications on groundnut yield with moderate disease pressure for all varieties. Yield increases of 400kg/ha were observed.

Rust

Rust is a fungal disease that usually appears at the same time as leaf spot (late in the season). It is characterized by orange-coloured pustules (spots) on the lower surface of leaves which rupture to release masses of reddishbrown spores. The lesions affect all above-ground parts of the plant except flowers. Infected leaves become necrotic and dry up; however, they remain attached to the plant. Rust in combination with late leaf spot can cause yield loss of about 30% to 40%. The disease appears sporadically in Malawi and is common in warm and humid low altitude areas and destructive in parts of Karonga, Mzuzu, Salima and the Shire Valley ADDs.

Farmers can minimize the incidence and severity of leaf spot and rust diseases by adhering to the following management practices:

• Plant a leaf spot- or rust-resistant variety, if available;

• Develop a sequence of crops in the rotation that are not hosts for the pathogens that cause early and late leaf spot (e.g., maize, sorghum, millet);

• Remove or bury infected crop residues;

• Remove and destroy infected volunteer groundnut plants that are present in rotation crops;

• Sow groundnuts early in the rainy season; and

• Apply appropriate fungicides in a timely manner, if they are available.

Groundnut Rosette Disease

Groundnut rosette is the most destructive disease in Malawi groundnut fields. The disease, which is transmitted by aphids, shows up in two forms in Malawi: chlorotic (yellow) and green rosette. Chlorotic rosette is characterized by yellow, twisted and distorted leaves while green rosette is characterized by mild mottling of younger leaves, older leaves appear dark green and reduce in size. In both forms of the disease, the plants are severely stunted and early infected plants do not produce any pods. The disease is present in all groundnut growing areas; however, it occurs sporadically. Total loss can result when the disease occurs in epidemic proportions.

Famers can reduce the impact of the disease by:

• Sowing early and at an ideal plant population (dense planting provides benefits);

• Removing and destroying all volunteer groundnut plants after harvest to reduce harboring aphids; and

• Using resistant varieties such as Nsinjiro (medium duration) and Baka (short duration).

Fungicide Guidelines for Suppression of Diseases

Seedling Diseases

Seed can be treated with the combination of thiamethoxam, metalaxyl-M, and difenoconazole to minimize the negative impact of soil pathogens on seeds and seedlings. This treatment can be especially important on soils that can remain wet after sowing.

Leaf Spot and Rust

Several fungicides are registered for use in Malawi after groundnuts have emerged. These are applied beginning 45 days after sowing and are often applied every two weeks until about two weeks before harvest. Three applications spaced 2 weeks apart will generally provide effective control of leaf spot and rust, but the fungicides available for use in groundnuts are protective and are not curative. They prevent lesions from forming, and for this reason, the first application should be applied no later than 45 days after sowing.

Fungicides available in Malawi include: azoxystrobin, boscalid, chlorothalonil, copper oxychloroide, mancozeb, maneb plus zinc oxide, propiconazole, pyraclostrobin, sulfur, tebuconazole, tebuconazole plus trifloxystrobin. These vary in effectiveness, expense, and availability. For leaf spot and rust disease, commercial fungicides that contain azoxystrobin, chlorothalonil, propiconazole, pyraclostrobin, and tebuconazole are the most effective. If a fungicide program of 2 or more sprays is used, rotate fungicides in order to minimize resistance and ensure the long-term viability of these products. For example, a spray program of azoxystrobin or pyraclostrobin followed by tebuconazole or propiconazole followed by chlorothalonil can be very effective in controlling these diseases and preventing resistance from developing.

Once lesions appear, treatment will not prevent the plant from shedding the leaves. Approximately three times as many leaves are infected as show visible lesions. For example, when 20% of leaves have lesions about 60% of the leaves on a plant are infected and will produce lesions and defoliate regardless of whether a fungicide is applied. For this reason, fungicides need to be applied early in the season to prevent epidemics from beginning. When leaf spot disease is widespread and severe in a field, a farmer may opt to harvest groundnuts before they reach optimum pod maturity. Protecting groundnuts from leaf spot disease so that canopy defoliation is minimized can give farmers more control over when to harvest. Using a variety with resistance to leaf spot or an effective fungicide program will increase yield because healthy plants will give the pods more time mature.

Impact of Nematodes in Groundnut Yield

Nematodes are small microscopic worms that can infect roots and pods causing galling, knots on the root that hurt the plant’s ability to move moisture and nutrients through its vascular system. There are many nonplant parasitic nematodes in soil that do not affect groundnuts. And, not all plant parasitic nematodes affect crops to the same degree. Nematodes prefer sandy, well-drained soils. The only way to suppress nematodes is through effective crop rotation as chemical control is cost prohibitive and marginally effective.

Impact of Vertebrates in Groundnut Yield

Birds and mammals consume groundnuts in the field and during drying and in storage. These pests can be difficult to control. Keeping groundnut fields free of weeds and establishing a buffer area around fields can reduce these pests. Harvesting in a timely manner can also decrease the length of time groundnuts are exposed to pest damage. In storage, place groundnuts in durable bags off the floor and away from walls to decrease hiding areas and entry points for vertebrate pests.