Rust Redux


Phakospora pachyrhizi (Asian soybean rust) arrived in the southern U.S. (Louisiana) in September 2004,  probably via spores swept along by Hurricane Ivan on its journey from South America. The disease, originating in Asia, has been established in Brazil, Paraguay, and parts of northern Argentina since February 2001.

With ability to infect and sporulate on a huge range of leguminous crops and weeds — including kudzu, pigeon peas, Centrosema, and Phaseolus — P. pachyrhizi infects over 95 species of plants representing at least 42 genera, including many wild and edible legumes.

The broad host range of P. pachyrhizi and P. meibomiar (its less aggressive New World equivalent), which is not found in most other rust fungi, increases the chances of winter survival, especially in the southern U.S. Generally, these alternative hosts are the primary focus for P. pachyrhizi epidemics, the disease appearing four weeks earlier than on soybeans. As such, they could become significant indicators of disease in the local environment, but by the same token may impact on disease intensity by acting as a reservoir of inoculum.

Spores And Symptons

Typical symptoms of Asian soybean rust on cultivated soybeans are tan to dark brown lesions mostly on leaves, but also found on petioles (leaf stalks), stems, and pods, with premature leaf fall and early plant maturation as disease severity increases. Lesions contain pustules called uredinia full of urediniospores — the common and so-called “summer” spores, wind-dispersed and produced in multiple cycles throughout the season. The much less common telia (pustules) and teliospores (so-called over-wintering spores) have been found on infected plants in Asia late in the season.

The sporulation pattern and potential of P. pachyrhizi shows why it spreads so rapidly, consolidates on the soybean crop, and is such a force to be reckoned with. Uredinia can develop within just five to eight days from leaf infection and may develop for up to four weeks after a single inoculation. Urediniospores appear within nine days of infection and are produced continuously for an additional three weeks. Secondary pustules arise at the edge of initial infections for another eight weeks, meaning first generation pustules can maintain sporulation for up to 15 weeks. This extended sporulation capacity allows the pathogen to persist with substantial inoculum potential, even under dry conditions, and re-establish when conditions are right for reinfection.

Plants are most prone to rust during soybean flowering stages R1 to R6, but susceptible to spore infection from the cotyledon stage onwards. Plants do not show any discernible symptoms for 10 to 11 days after infection. What’s more, the disease is difficult to detect in the early stages, because lesions look like mere pinpricks on the leaves and require a 10X to 20X hand lens magnification to be clearly seen.

Add all this uncertainty and delay together and throw in ideal conditions for disease development and spread and at least six hours of leaf surface wetness (10 to 12 hours optimum) from rain or overhead irrigation with a temperature of 47Ëš to 60Ëš F, and unsprayed fields can be wiped out.

There is nothing obvious in soybean germplasm to offer immediate and meaningful resistance to Asian soybean rust. Brazilian cultivars are highly susceptible. Development of resistant cultivars is an ongoing but long-term business still years away. It requires a molecular approach to the development of gene libraries of P. pachyrhizi and P. meibomiae, and development of a molecular understanding of pathogenicity (virulence).

Projected Loss Of Crop

No one is absolutely sure what the total loss to U.S. growers could be. But if the experience of growers elsewhere is anything to go by, they will amount to very big bucks indeed. In southern Africa, there have been cases of total crop loss, with pods completely shriveled and destroyed. Reports from Brazil suggest 50% loss for untreated crops, but only 10% for those sprayed with fungicides.

Back in 1984, a far-sighted economic risk analysis projected potential U.S. losses from soybean rust at $7.1 billion. USDA Task Force current-day projections are more than twice that figure due to increased production and more soy-derived, value-added products. The average worst case scenario is likely to be 40% to 50% loss, and at best a late-season rust attack would cause just 5% to 10% loss. The USDA projection of a 9.5% drop in output, provided fungicides are used, still equates to a staggering $2 billion loss for U.S. soybean growers. 

Fight With Fungicides

With resistant varieties years down the line, fungicide treatment is the only option for disease management. So far, almost all EPA-approved products for soybean rust belong to either the triazole and strobilurin groups of chemicals. This could pose future risks of insensitivity (resistance) in the Asian soybean rust population, if fungicides with similar structural chemistries and the same single site mode of action are used too often and for too long. But by the same token, mixtures of fungicides of different chemistry, mode of action, and target site should enhance control as well as impede development of resistant strains of the fungus.

Contact (protectant) fungicides (e.g. chlorothalonil) only act by inhibiting and preventing spore germination. Effective control with these fungicides therefore requires complete coverage of all susceptible leaves and surfaces and before any spores alight on the crop. Contact fungicides used in Brazil have given variable results and are only applied under low disease pressure — or preferably, before the disease arrives. One strategy involves a contact fungicide spray at the late vegetative stage (V5) to protect the lower leaves, followed by sprays of systemic fungicide during the reproductive stages (R1 through R5).

Triazole and strobilurin fungicides with, respectively, truly systemic or locally systemic (translaminar) activity are generally preferred due to improved flexibility and activity. And coverage, while still important, is not so crucial for systemically active fungicides. They penetrate the leaves and move within the plant to offer at least some eradication of infection inside the leaf tissue. Movement of truly systematic fungicides in the plant is upwards and outwards in the xylem (water transport) tissue, so repeat sprays are required to ensure protection of new growth produced since the previous application.

Triazoles, for example, are both contact and curative in action with a generally longer residual activity than most other fungicide chemistries, with ability to arrest established Asian soybean rust infections. On contact with growing tips of fungal hyphae filling the intercellular spaces, the triazole molecule destroys new fungal growth and prevents branching.

Spray Application Choices

Narrow width planting in Brazil has been abandoned in favor of relatively wide rows of between 45 to 50 centimeters to accommodate fungicide spraying. This is claimed to allow better droplet penetration into the canopy and reduce the build-up of warm humid microclimates that favor soybean rust. But by the same token, it also significantly reduces the number of plants per acre and therefore lowers the ceiling on potential yield.

Ground spraying at this stage of crop growth and development is likely to cause considerable wheel damage, estimated at some 3% to 5% of the crop, and additional losses if the sprayer has to pass over rows of soybean plants. Growers will not be able to use any old sprayer, but instead dedicated high-clearance machines and rigs that may not be immediately available, and therefore require one high-clearance sprayer to cope with every 5,000 to 8,000 acres of soybean crop.

There are other potential problems, too. Epidemiological studies show that soybean rust develops and spreads most quickly during periods of prolonged wetness — the very time when sprayers are likely to get bogged down in wet fields.

Growers need speed spraying to protect the entire planted crop before rust has the chance to establish, even in a single field. This is particularly important when a correctly timed follow-up spray is required. USDA estimates are $25 to $35 per spray, which includes materials and application fees. Those in the Midwest are expected to spray once or twice, waiting at least 14 to 20 days before the follow-up application.

Smaller droplets from air-assisted ground rigs may be the answer for improved canopy penetration and better under-leaf cover. But these will still not satisfy the need for sufficiently fast spraying to protect the whole soybean crop on one farm in the same time frame when most at risk.

So what is the answer? Aerial spraying has already proved its worth in South America for speed and fast coverage of huge areas, and the ability to overcome the constraints of planting pattern (e.g. narrow row width) and plant growth development (tall plants at the R stages) on the use of ground sprayers. Growers can maintain narrow width plantings with higher plant densities per acre and higher yield ceilings by making maximum use of the available land. And lower canopy coverage is achieved with small droplets delivered in low volumes harnessing natural air currents to penetrate the crop canopy and target these most vulnerable older leaves.

Aerial spraying avoids the risk of disease spread by spores released from lesions and pustules by ground sprayers as they move through the crop, and the transfer of rust infection via spores from one field to the next and between different farms where spray contractors are used.

Speed of spraying and ability to cover large areas in a short space of time as achieved by aerial application is clearly the only option for stemming the tide of Asian soybean rust in the face of a worst case scenario. But whatever the disease holds in store for North America, U.S. growers appear undeterred in their loyalty and confidence in the soybean crop.

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