Fig. 3 Uredia of Phakopsora pachyrhizi rupture the epidermal cells releasing numerous uredospores (SEM photo by Eve du Preez - KZN-DAEA)

Pat Caldwell and Mark Laing
Discipline of Plant Pathology, School of Applied Environmental Sciences, University of Natal, Private Bag X01, Scottsville 3209

Soybean rust, caused by the fungus Phakopsora pachyrhizi, is a new disease of soybeans in South Africa (SA). Although common and extensive in the Far East, soybean rust was only reported in SA for the first time in March, 2001. It appeared again in March, 2002. We can now assume that the pathogen is an established threat to soybean production in SA.

First reports

First reports of this pathogen were in 1902 in Japan. By 1914 it appeared in several other eastern countries with an epidemic occurring in south-east Asia [1]. When Australia began to expand its soybean industry, the pathogen appeared but it has never reached epidemic proportions. Only in the late 1990s was it reported in Africa. It is thought that the pathogen was wind-borne from Asia to Africa. It was initially identified in Uganda, Kenya and Rwanda and in January 1998 was identified in Zimbabwe. The impact on the soybean crop in Zimbabwe was devastating and yield losses in commercial crops ranged between 60-80%. Asian soybean rust has not yet been reported in the United States (US), or South America, but is likely to arrive there in the next 5-10 years.

Symptoms

Early symptoms appear as small water-soaked lesions, which must not be confused with spider mite symptoms (Fig. 1) [1].

Fig. 1 Early chlorotic symptoms of soybean rust on the upper surface of leaflets (Rikus Kloppers - PANNAR).

Lesions enlarge, resulting in leaf senescence. Lesions change from grey to tan or brown in colour and are restricted by leaf veins. Lesions are found mainly on the leaves where they are most common on the under-surface (Fig. 2). However, in severe cases, lesions can also be found on pods, stems and petioles [1].

Fig. 2a & b. As the season progresses numerous pustules, releasing uredospores, appear particularly on the abaxial surfaces of leaves (Rikus Kloppers - PANNAR).

Another identifying characteristic are the clouds of spores (uredospores) that are visible as one moves through the field. Pustules are more visible on the lower than upper surface of the leaf, and exude clumps of spores (uredospores) from uredia (Fig. 3, see top of page). Colour of pustules varies and appears to depend on age and the interaction between soybean genotype and race of the pathogen.

Yield losses

Once lesions appear, defoliation is rapid resulting in fewer pods and seeds, lower seed weight and early maturity (Fig. 4) [2] . Yield losses in SA in 2001 were reported to be in the region of 10-80%. In some areas, losses of up to 100% have been reported if monocropping, with no rotations, is practised.

Fig. 4 Defoliation is rapid resulting in early maturation and reduced yield (Rikus Kloppers - PANNAR).

Host range

Soybean rust is an obligate parasite and survives over winter and unfavourable conditions on alternate hosts. The host range of this fungal pathogen is large - 95 different species in 42 genera in the bean family, Fabacea [3]. Since rust may persist on these hosts throughout the year, they may act as inoculum reservoirs. Alternate hosts have not yet been identified in SA and Zimbabwe but they will influence the potential for the pathogen to become endemic.

Spore survival

Literature indicates that uredospores may survive as long as 50 days. It is not known if the pathogen produces teliospores (sexual spores) and whether there is a secondary host which these spores infect during the disease cycle in SA.

Development

Important factors for promoting infection are temperatures of 15-28°C, relative humidities of 75-80%, with precipitation and long periods of leaf wetness [4]. Moisture is essential for uredospore germination and development, so mistbelts with heavy dews, as found in many parts of KZN, will provide suitable conditions for infection.

Unlike many pathogens which must find stomata, wounds or some opening before they are able to penetrate the host, soybean rust uredospores are able to penetrate directly through the leaf cuticle and epidermis, making infection easier and quicker (Fig. 5) [5]. The latent period is about 9-10 days.

Fig. 5 Unlike most fungi, Phakopsora pachyrhizi is able to penetrate directly through the cuticle and epidermis of the host (SEM photo by Eve du Preez - KZN-DAEA).

Control

Genetic engineering does not offer much potential for the control of soybean rust. Biocontrol could possibly be effective using Urocladium and Sphaerolopsis. Increasing phosphorus levels has been found to reduce incidence of soybean rust . On a practical front, irrigation should take place either in the middle of the day so leaves can dry before dew sets in, or at night so as not to extend the dew period.

However, in Zimbabwe, soybean rust has been effectively controlled by spraying with appropriate fungicides. All registered products for use in SA belong to the sterol biosynthetic inhibitors (SBI's). Fungicides registered to control soybean rust in SA have received emergency registration largely on the strength of their use in Zimbabwe and their use on other field crops, e.g., beans.

Research is underway to evaluate the effectiveness of fungicides under local conditions and to refine dosage rates and application timing of the emergency registered, and other, fungicides. It will also be important to evaluate fungicides belonging to other chemical groups to enable these fungicides to be used in mixtures or in alternative programmes with existing fungicides. Rates of application with length of control will also be evaluated this summer.

Conclusion

SA produces 208 000 tons of soybean seed on 193 000 hectares of land. In 2001, 523 000 tons of soybean oilcake meal was imported into SA, valued in the region of R1.1 billion. Soybean yield losses of up to 70% are possible, and if these did materialise, millions of rands will have to be spent on importing additional oilcake meal.

A number of organisations, including the Department of Agriculture and Environmental Affairs, the Protein Research Trust, the Agricultural Research Council and private companies, together with the University of Natal, are all involved in collaborative research programmes to find solutions to this devastating disease on soybeans.

The development of resistant varieties will take several years. In the meantime, fungicides provide the short-to-medium term solution to the soybean rust problem. Farmers are encouraged to watch their crops carefully and to begin spraying, using the recommended fungicides at the registered rates, as soon as disease symptoms appear to ensure a high-yielding crop this season.

Literature

1. Sinclair, J.B. & Backman, P.A. 1989. Compendium of soybean diseases. Third edition. APS Press, Minnesota, USA.
2. Yang, X.B., Tschanz, A.T., Dowler, D.W. & Wang, T.C. 1991. Development of yield loss models in relation to reduction of components of soybean infected with Phakopsora pachyrhizi. Phytopathology 81: 1420-1426.
3. Hartman, G.L. Wang, T.C. & Hymowitz, T. 1992. Sources of resistance to soybean rust in perennial Glycine species. Plant Disease 4: 396-399.
4. Kochman, J.K. 1979. The effect of temperature on development of soybean rust (Phakopsora pachyrhizi). Austr.J. Agric. Res., 30 :273-277.
5. Koch, E., Ebrahim-Nesbat, F. & Hoppe, H.H. 1983. Light and electron microscopic studies on the development of soybean rust (Phakopsora pachyrhizi Syd.) in susceptible soybean leaves. Phytopathology 106: 302-320.