What Is Soybean Rust? Symptoms and Control Measures

Soybean rust is a fungal disease impacting soybean production worldwide. Caused by two fungal species, Phakopsora pachyrhizi and Phakopsora meibomiae, it manifests as lesions on the leaves that can lead to plant death and yield reductions. First identified in Japan in 1902, the pathogen has since spread across multiple continents. The more aggressive species, P. pachyrhizi, was confirmed in the continental United States in 2004 and has established a presence in southern states.

Identifying Soybean Rust Symptoms

The first symptoms of soybean rust appear on the lower leaves in the plant’s canopy. The disease presents as small, gray to tan or reddish-brown lesions on the leaf surface. These lesions are angular, as their shape is restricted by leaf veins, and can grow to be 2 to 5 millimeters in diameter. As the infection progresses, these spots become more pronounced on both leaf surfaces.

A diagnostic feature is the development of raised pustules, known as uredinia, within the mature lesions. These pustules are most commonly found on the underside of the leaves and have a volcano-like shape. When ruptured, they release powdery, tan to reddish-brown spores. In advanced infections, symptoms can also be found on the plant’s petioles, stems, and pods. The visual symptoms caused by both fungal species are nearly identical, making field differentiation difficult without laboratory analysis.

The Soybean Rust Pathogen and Its Spread

The fungi causing soybean rust are obligate parasites, requiring living plant tissue to survive and reproduce. The disease cycle is driven by the dispersal of urediniospores. This cycle begins when windblown spores land on a susceptible soybean leaf under conditions of prolonged wetness and high humidity, ideally between 75-80%. The pathogen penetrates the host tissue, and after 9 to 10 days, forms the volcano-shaped uredinia on the leaf surface.

These uredinia produce new spores for several weeks. The spores are lightweight and carried by wind currents, allowing for rapid, long-distance dispersal, such as the pathogen’s journey to the United States via hurricane winds. Favorable temperatures for infection range from 15 to 28°C (60-82°F). The pathogen survives between growing seasons on alternative hosts, like the invasive weed kudzu, which helps establish a presence in warmer regions.

Impacts of Soybean Rust on Agriculture

A soybean rust outbreak causes substantial reductions in crop yield. In regions where the disease is common, yield losses can range from 10% to 80% in untreated fields. Under optimal conditions, the severity of an infection in a field can double every few days. The fungus harms the plant by damaging leaf tissue, which disrupts photosynthesis and leads to premature defoliation, fewer pods and seeds, and reduced seed weight.

Beyond direct yield loss, the disease increases production costs for farmers. Management requires diligent scouting, and control often necessitates multiple applications of fungicides, adding considerable expense. These additional costs cut into the profitability of soybean farming. The economic effects extend beyond individual farms, as widespread outbreaks can impact regional economies. These impacts can ripple through related industries, and significant production losses can affect international commodity prices.

Controlling Soybean Rust Outbreaks

Managing soybean rust relies on an integrated approach combining chemical, cultural, and genetic strategies. The most common method for direct control is applying foliar fungicides. These treatments are most effective when applied preventatively or at the first sign of infection, making timely detection through scouting important. Both protectant and systemic fungicides are used to combat the disease.

Planting soybean varieties with genetic resistance is another management tool. While resistant varieties are available, the pathogen’s ability to develop new races that overcome specific resistance genes presents an ongoing challenge for plant breeders.

Cultural practices also play a role in reducing outbreak risk. Managing alternative hosts and volunteer soybean plants is important because these plants serve as a reservoir for the pathogen between seasons. Crop rotation has limited effectiveness for a single field against a wind-dispersed pathogen but contributes to regional disease management.