3.3.5
DETECTION OFPHYTOPHTHORA SOJAE FROM SOIL WITH SOYBEAN LEAF-DISC BAITING

PENG JINHUO1 and T ANDERSON2

1Dalian Animal and Plant Quarantine Bureau, Dalian, 116001 PR China; 2Greenhouse and Processing Crops Research Centre, Harrow, Ontario NOR 1G0, Canada

Background and objectives
Phytophthora sojae, the pathogen of soybean root and stem rot, is one of the quarantine organisms of PR China. A large amount of soybean is imported each year from areas where this disease has been reported. Some of the factors concerning the detection of this pathogen from soil using soybean leaf-disc baiting methods have been reported previously [1], but the method is difficult to use in routine quarantine detection. Our objective was to develop an effective technique for the detection of this quarantine organism.

Results and conclusions
A high detection rate of P. sojae was obtained when air-dried, naturally infected soybean field soils in a flask were moistened and pre-incubated under lights at 22-26C for 4-6 days, flooded with 5-10 mm of distilled water, and then baited with soybean leaf-discs for 6-24 h. Distilled water was adopted to incubate the baited leaf discs. Sporangia emerging from the edge of the infected leaf discs were observed under stereo microscopy after 72 h incubation in distilled water. This technique was more sensitive for the detection of P. sojae from soil than previously reported techniques. For pure isolations, zoospore solution was spread on 1.5% water agar containing anti-bacterial antibiotics, and 24 h later the germinated zoospores were isolated and pure cultures could be obtained. When baiting leaf discs were from a plant with a special resistance gene, only the pathogens which could overcome this gene could be baited and detected. With this technique, the pathogen has been detected several times in soybean shipments.

References
1. Canaday CH, Schmitthenner AF, 1982. Soil Biology and Biochemistry 14, 67-68.