THE OCCURRENCE OF ASIAN (CAUSED BY PHAKOPSORA PACHYRHIZI) ON COMMON BEAN IN SOUTH AFRICA

MM Liebenberg^*, AJ Liebenberg^ and ZA Pretorius^ ^ARC-Grain Crops Institute, Private Bag X 1251, Potchefstroom 2520; ^Depaitment of Plant Sciences, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa [email protected]

INTRODUCTION Phakopsora pachyrhizi was first reported on (Glycine max) in South Añica in 2001 (Pretorius et al, 2001). However, none was found on until April 2004, when it occuiTed on a small patch oí Ph. vulgaris (sun^ounded on tliree sides by heavily infected soybeans) at Cedara Agricultural Research Station near Pietermaritzburg, South Africa (Du Preez et ai, 2004). At this time, no P. pachyrhizi could be found on common bean in trials ± 1 km down wind of the soybeans. A trial was therefore planted to obtain information on the possible threat of Asian to the common bean, to detemiine differences in susceptibihty between Ph. vulgaris genotypes, and to identify potential sources of resistance.

METHODS Field Trial The 91 entry, single row tiial, included a wide spectiixm of Ph. vulgaris gemiplasm containing all known (RR) genes conveying resistance to bean rust (caused by appendiculatus) as well as breeding lines with multiple RR genes and the South AMcan National Dry Bean Cultivar Trial (30 entries). Important additional USA genotypes were contributed by Drs. JR Steadman (University of Nebraska) and MA Pastor-Corrales (USDA, Beltsville). It was planted at Cedara Research Station (latitude: 29.5 south; longitude: 30.3 east; altitude: 1076 m above sea level) on 17 January 2005. Each Ph. vulgaris row was flanked by a row consisting of a mixture of short and long season soybeans (Stork, LS444, LS677), planted on 9 December 2004, and all highly susceptible to P. pachyrhizi. No fungicides were applied.

Greenhouse inoculations were done at the University of the Free State (UFS), Bloemfontein, and at the ARC-Grain Crops Institute (ARC-GCI), Potchefstroom, South Afiica. At UFS, an isolate collected from and increased on soybeans was used. Fresh (in water + Tween 20) were apphed to both leaf surfaces of seedling plants using an atomizer. Plants were incubated for 16 h incubation at 21 °C, >95 % relative humidity (RH), then removed to a greenhouse at 16/25 °C night/day. Consistent results, but generally with low severity, were achieved. Better results were achieved with Soltrol 130 as earner, but this caused leaf bum. In addition, inoculation of PA. vulgaris with dry spores of P. pachyrhizi in a settling tower gave satisfactory results. Exposing primary leaves of the common bean cultivar Teebus to 16 mg spores for a 3 min, followed by a high-humidity period as described above, provided sufñcient rust lesions for rating purposes.

At ARC-GCI, an unpurified sample collected from the common bean, but increased on soybeans, was used. Only isolated pustules were obtained on G. max after transfer of the rust from common bean leaves, and only faint necrotic flecking with no pustules was obtained on common bean. Two increases on soybean were necessary to obtain sufficient spores for inoculation. Results using spray inoculations were unsatisfactoiy. The best method was application of a high concentration fresh dry spores using a No.l artist's paint brush on mature leaves wetted with

49 0.01% Tween 20 in tap water. Plants were incubated for 24 h at >95 % humidity at ±19 °C and then removed to a greenhouse at ±20/24 °C night/day. Well-developed sporulating pustules were observed 18 d after inoculation. At both the UFS and ARC-GCI, pustules appeared gradually and considerably later than on soybeans.

RESULTS AND CONCLUSIONS Field ratings (summarized in Pastor-Corrales et al, this edition) ranged between 0 for the upper canopy to 8 for the lower canopy, lines with a deteminate growth being the most seriously affected. Serious defoliation oí Ph. vulgaris can take place as a resuk of infection by P. pachyrhizi. This is accompanied by both chlorosis and necrosis. However, serious infection of Ph, vulgaris by P. pachyrhizi is dependent on the close proximity of heavily infected soybeans, the relative timing of the mst epidemic and planting date, and the presence of ideal conditions, in particular high humidity. Leaf maturity may also play an important role in the field. Ph. vulgaris may not be able to maintain the epidemic in the absence of G. max as a source of inoculum, and spore production of P. pachyrhizi on Ph. vulgaris appears to be lower than on G. max. There were also fewer pustules per unit area, and P. pachyrhizi appeared to have a less adverse effect on yield oí Ph. vulgaris. The viabihty of P. pachyrhizi may be adversely affected by Ph. vulgaris. At this stage, P. pachyrhizi is probably not a serious threat to Ph. vulgaris under normal fanning conditions, unless unusually high levels of inoculum are present, ideal conditions for soybean rust prevail, Ph. vulgaris is planted adjacent to heavily infected soybeans, and P. pachyrhizi is not controlled on the soybeans. Differences in the reaction of PA. vulgaris entries appeared to be a function of maturity at the time of infection rather than differences in genetic resistance to P. pachyrhizi, and no resistance appeared to come from "Í7r"-genes. However, cooler temperatures and lower rainfall (but consistent nightly high humidity) that may have affected levels of P. pachyrhizi, were experienced after defoliation of the soybeans. Due to the high variability of P. pachyrhizi, the danger does exist that the pathogen may adapt to Ph. vulgaris and become a more serious threat, especially in countries such as Brazil, the USA and South Africa, where G max and Ph. vulgaris are cultivated in the same area, and where P. pachyrhizi can overwinter.

Future plans A simplified, rephcated version of the Ph. vulgaris gemiplasm trial is planned for 2005/6, using only a few selected entries. Two planting dates for soybeans will be used in order to provide inoculum pressure over the full growth period of common beans. Miniature trials may be included to test a) the ability of common bean to maintain the epidemic and b) the effect of a single fungicide application. Monitoring oí Ph. vulgaris breeding trials will be continued for signs of damage due to P. pachyrhizi. Greenhouse inoculations will be continued and optimized for screening of common bean genotypes. Other possible areas of study include latent period, pustule fomiing, sporulation, viability, and the effect of P pachyrhizi on yield oí Ph. vulgaris.

References

Du Preez, ED, Van Rij, NC, & Lawrance, KF. 2004. First report of soybean rust caused by Phakopsorapachyrhizi on dry bean sin South Aft-ica. Plant Disease 89:206. Pretorius, ZA, Kloppers, FJ and Frederick, RD. 2001. First report of soybean rust in South Africa. Plant Disease 85:1288.

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