Pablo Moreno of Moreno Brothers in Pergamino, Argentina making an application over soybeans in an Air Tractor AT-502 using nine AU5000 Micronair rotary atomizers with an application rate of two gallons per acre (20 l/ha) and a swath of 90 feet (28 meters).


Soybean rust control with Micronair rotary atomizers

By John Clayton
Micron Sprayers Ltd
England.
The arrival of Asian Soybean Rust (ASBR) for the first time late last year into the Southern U.S. has caused alarm amongst soybean growers and prompted action by the U.S. Environmental Protection Agency (EPA), the U.S. Department of Agriculture (USDA) and state departments to plan for a potential major outbreak this coming season. Soybean production in the U.S. accounts for around 16% of all U.S. agricultural production. The acreage planted this year is expected to be around 74 million acres (almost 30 million ha).

Asian soybean rust, Phakopsora pachyrhizi, is a particularly virulent fungal strain that has the potential to decimate the soybean crop with potential yield losses of between 50 to 80% within one month of infection, if left untreated. The disease is spread mainly by the wind and is believed to have been carried into the U.S. from South America during last year’s hurricanes. The disease, which originates in Asia, has become established in Brazil, Paraguay and parts of northern Argentina since February 2001.

By 2003/4 season, ASBR was found in most soybean crops in Brazil with estimated yield losses of 2.2 million metric tonnes (MMT) or 5% of annual production at a cost of some $1.2 billion USD. This year’s estimates of losses (2004/5 season) were even higher, around 4.5 MMT due to ASBR. The disease has been found in 81 municipalities in Brazil this year, spread across eight major soybean producing states in the country. In 2004/5 season, 95% of Brazilian soybeans were sprayed, most two times, although some fields received up to four applications.
The presence of ASBR has now been recorded in a number of U.S. states, including Arkansas, Florida, Georgia, Louisiana, Missouri, South Carolina and Tennessee. The disease can infect over 95 species of plants, including wild and edible legumes. The weed Puerania labatz (kudzu) that grows along roadsides and ditches, for example, can act as a host plant for the disease to infect soybean crops. The disease may over winter on living plant material if temperatures stay above freezing and can spread rapidly when the host plant releases air borne spores that are carried by the wind.

Disease symptoms

Typical symptoms of ASBR infection on cultivated soybeans are tan to dark brown lesions, mostly on leaves, but also found on petioles (leaf stalks), stems and pods. These lesions contain pustules that release spores that are produced in cycles throughout the growing season. These spores can develop within nine days from leaf infection and are produced continuously for three weeks. Plants are most prone to rust during soybean flowering stages R1 to R6, but can be infected any time from when the plants emerge. Plants do not show any discernable symptoms for 10-11 days after infection. Optimal conditions for developments of the disease are when the leaves have been wet for at least 6 - 12 hours and temperatures are between 60-80°F.

Fungicides for rust control

In the absence of resistant cultivars, fungicide application will be the cornerstone to prevent serious crop failure in soybeans. Whilst it is still unclear how far the disease will spread in the U.S., as climatic conditions vary from those in Brazil, vigilance is required and timely intervention with fungicide applications will be essential to prevent major crop damage should the disease become established. To counter the threat from ASBR, many U.S. states have authorised a number of new fungicide products for use in soybeans under Section 18 of the EPA emergency quarantine exemptions for pesticide registration. Section 3 products already have full clearance for use. See Table 1.

Table 1: Fungicide Options for Asian Soybean Rust Control in the US (source: EPA 3/31/05)
Chemical Group
Product
Active Ingredient(s)
Manufacturer
Registration
Status*
Chlorothalonil
Bravo
chlorothalonil
Syngenta
Section 3
Echo
chlorothalonil
Sipcam Agro
Section 3
Strobilurins
Headline
pyraclostrobin
BASF
Section 3
Pristine
pyroclostrobin+ boscalid
BASF
Section 3 pending
Quadris
azoxystrobin
Syngenta
Section 3
Triazoles
Domark
tetraconazole
Valent U.S.A
Section 18
Folicur
tetraconazole
Bayer
Section 18
Lardedo
myclobutanil
Dow AgroSciences
Section 18
Bumper
propiconazole
MANA
Section 18
PropiMax
propiconazole
Dow AgroSciences
Section 18
Tilt
propiconazole
Syngenta
Section 18
Combinations
Quilt
azoxystrobim + propiconazole
Syngenta
Section 18 MN only
Stratego
propiconazole + trifloxystobin
Bayer
Section 18
Headline SBR
pyraclostrobin + tebuconazole
BASF
Section 18 MN only
• Registration status varies according to State.
• This list is not exhaustive as more products may be added – check latest situation with your local state department.


Soybean rust fungicides are classified as protectant or curative according to their mode of action. Chlorothalonil products are used entirely as protectants and therefore require good coverage of all plant surfaces prior to infection, including upper and lower leaf surfaces. As such, they are generally used early season, however, as the crop grows and becomes established, it becomes more difficult to achieve complete coverage on all plant surfaces with sprays whether applied from the ground or air.

Strobilurins are systemic in their activity being able to penetrate within the plant and therefore help protect against initial infection and subsequent penetration of the fungus within the plant surface. Again, good coverage is required, but the systemic nature of strobilurins means under leaf coverage is less important as the chemical can pass through the leaf.

Similarly, the triazoles are systemic in their activity and also are absorbed into the plant – generally more readily than the strobilurins. Movement of fungicides within plants is always upward and outward as they move through the water transport system (xylem), so often multiple applications will be required during the season to protect new growth. Triazoles are both preventative and curative in their action being able to arrest the development of the disease, if the soybean is already in the early stages of infection.

Heavily diseased plants will not be saved if fungicides are applied too late. Again overall coverage is less important than with protectant-only fungicides due to the systemic activity of the triazoles. However, timing of application is critical with all fungicides. Each chemical group has a discrete site of action on the disease lifecycle, so different products will be used at different times. Often, combinations of different actives applied together will be used to control the disease more effectively.

The Brazilian experience

In Brazil, where the disease has spread rapidly, much of the soybean crop is sprayed with aircraft so U.S. farmers and aerial operators may be able to benefit from the experience already gained from Brazil. The Brazilian soybean crop is about half the planted area of the U.S. (around 36 million acres or 15 million ha). The vast area of soybeans planted, often in remote areas, lends itself to aerial application. As most ag-aircraft in Brazil are Ipanema-typewith medium payload capacity (750 kg), then volume rates need to be as low as possible.

Recent spray trials conducted by Professor Ulisses Antuniassi and his team at the Sao Paulo State University (UNESP), Botucatu, in conjunction with FMT (Fundação Mato Grosso) – the main soybean growers association) compared different application techniques and their efficacy in controlling soybean rust. The first series of trials compared ground application with high clearance rigs applying volume rates of 120 l/ha (around 13 gals/acre) using hollow cone and flat fan nozzles and with an aerial application at 30 l/ha (around 3.3 gals/acre) using hollow cone nozzles.

At the same time, they also compared these ‘conventional’ application techniques with rotary atomisers applying between 5 - 12 l/ha (0.5 – 1.4 gals/acre). In this instance, a 10% oil adjuvant mix was also used to minimize evaporation of spray droplets and improve retention and absorption of fungicides on plant surfaces.

Two fungicide applications were made, the first with a strobilurin + triazole combination and the second with a single triazole application (myclobutanil). There was little difference between any of the applications – despite the huge variation in volume rates applied - all gave excellent control of rust compared with the control plots.

This February (2005) in the Mato Grosso state in central Brazil, further trials were made in conjunction with UNESP and FMT to test different aerial application systems. Micronair AU5000 rotary atomisers were tested at 10l/ha (with 10% adjuvant oil in water) and 20 l/ha (no oil). Again excellent results were achieved with an average of 96% and 88.9% rust reduction respectively. Interestingly, the addition of oil improved the control level slightly in these tests although this was not statistically significant.

Three fungicide applications were made, the first with myclobutanil, the second tebrucanozole and the third with flutriafol – dichlofluanid at 14- and 24-day intervals. In Brazil protectant fungicides are often applied at the late vegative stage (V5) before flowering to protect the lower leaves followed by sprays of systemic fungicides during the reproductive (R1-R5) phase of growth

The current U.S. situation

As Asian soybean rust has only just been discovered and previously U.S. soybean farmers applied relatively few fungicides to their crops, experience of best application methods is limited. Good penetration and coverage of sprays are important (even with systemic fungicides), so selection of the appropriate drop size and spray volume is important, but timing of sprays is everything.

Ground rigs will struggle to keep pace with the area to be sprayed if the disease takes hold. Aerial applications will be pivotal to sustain the soybean crop. Ground rigs can also damage the crop (estimated at 3-5% of crop), particularly when the fields are wet and can spread the disease as they move from one farm to another. Specialized high-ground clearance rigs will be necessary to apply fungicides late in the season and therefore many farmers will be ill equipped to contain the spread of such a virulent fungus. Typically two, or possibly three, fungicide applications may be required during the season. The USDA estimates $25-35/acre chemical costs and application fees for most growers with sprays usually applied at 14-20 day intervals. Aerial application may therefore be the only option for many growers. The demands on aerial operators at the peak of an outbreak, at times could exceed the industry’s capacity. As it is critical sprays are applied in a timely manner, high productivity and good spray coverage will be essential. See Figure 1.


Current U.S. label recommendations for many soybean fungicides advise application volumes typically around five gals/acre for aerial application and 10 gals/acre for ground rigs. The dose rates of fungicide applied are the same for both – only the water volume is reduced for aerial application. These volume rates are largely based on the desire to obtain penetration of the crop canopy, yet maintain coverage over all plant surfaces using typically fine to medium-sized sprays with standard pressure nozzles. Swath widths should be carefully chosen to avoid any missed application, as unsprayed areas will be an important source of infection. A minimum flying height of 10-12 feet is usually recommended to ensure good fungicide distribution.

Rotary nozzles offer the benefit of improved control of droplet size ensuring good coverage and penetration of the soybean plant at lower spray volumes to improve operator productivity. Figure 1 illustrates the productivity of a fixed-wing aircraft at different volume rates demonstrating a 40% increase in acres sprayed by reducing rates from five to two gals/acre.

As results in Brazil have demonstrated, lower spray volumes of one-three gals/acre give excellent results. In time, lower volume rates of say two-three gals/acre for aerial application may become the norm as industry experience in controlling this disease improves. From experience in Brazil, some U.S. fungicide suppliers already advise growers to use an oil adjuvant or surfactant to enhance uptake of the fungicide. Selecting the most appropriate fungicide mix, adjuvant, drop size and volume rate to improve productivity, and hence timing of application, may well offer the best solution to improved disease control.

U.S. aerial operators, in the coming months, may have a pivotal role in defending the soybean crop from this invasive and virulent plant disease. Be prepared.

Useful web sites on soybean rust:
www.rustready.com
www.ipmcenters.org/newalerts/soybeanruts
www.sbrusa.net
www.aphis.usda.gov/lpa/issues/sbr/sbr.html
www.epa.gov
www.usda.gov/soybeanrust
www.oardc.ohio-state.edu/soyrust/index.htm
www.agaviation.org
www.apsnet.org/online/feature/rust