lDepartment of Plant Protection, Huazhong Agricultural University, Wuhan, 430070, P R China; 2Department of Plant Science, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada

Background and objectives
The success of using hypovirulent factors in Cryphonectria parasitica to control chestnut blight in some areas has been stimulating scientists to exploit this kind of factor in other pathogens. In Sclerotinia sclerotiorum, Boland [l] reported isolate 91 was hypovirulent, and transmissible on PDA medium. Li et al[2] isolated a naturally-occurring abnormal isolate designated as EP-1 PN from eggplant in China. It was weakly pathogenic to several crops. The recent study showed that Ep-1 PN could transduct its hypovirulence to some virulent isolates of S. sclerotiorum. The objective of this study was to investigate the transmission of the hypovirulent factor in EP-1 PN on canola.

Materials and methods
Two experiments were conducted. First one was to test the effect of EP-1 PN density on transmission, whereas the second one was for investigating the transmission durability on canola leaves. For both, EP-1 PN was cultured at 20-22 C for 10 days with PDA. Then it was blended with Waring Blender. In the first experiment, the hyphal suspension was diluted by 0, 5, 10, 100, 1000 and 10 000 times with sterile distilled water, respectively, each of which was sprayed onto the five pots of canola plants (Westar) with water as control. The agar plugs with EP-1PNA5, a virulent isolate, were inoculated on the leaves. The plants were kept in mist chamber at 22/15 C (day and night). Lesions initiated by EP-1 PNA5 were scored 24h after inoculation. Sclerotia formed on plants were detached 10 days later, after being surface-disinfected, they were put on PDA medium. When the colonies appeared, they were scored as normal and abnormal colonies. In the second experiment, the hyphal suspension was diluted by 2 times and sprayed onto canola plants with spraying water as control. 0, 1,2,3,4,5, and 6 days later, EP-1 PN agar plugs were inoculated on each of five pots of plants, respectively. Disease initiation and scoring were the same as above-mentioned.

Results and conclusions
In the first experiment, EP-1 PN could significantly inhibit the lesion expansions initiated by EP-1 PNA5 in the treatments of 1, 1/5, 1/10 and 11100 hyphal suspension sprayings, the lesion diameter of which was 2.15, 2.17, 2.53 and 4.26 cm, respectively , after 120 h, whereas 1/1 000 and 1/1 0000 hyphal suspension treatments had lesion diameters of 8.16 and 9.6 cm, respectively. The control lesions reached to 10 cm in diameter after 120 h. Thus EP-1 PN could transduct its hypovirulence obviously in high hyphal concentration treatments. Ten days after inoculation, 70 sclerotia formed in control plants, whereas in 1,1/5, 1/10, 1/100,1/1 000 and 1/1000 hyphal suspension treatments, 0, 0, 0, 2, 12 and 38 sclerotia were recorded. On PDA medium,80% of the control sclerotia produced normal colonies, whereas all the sclerotia from EP-1 PN treatments formed abnormal colonies, indicating that transmission of EP-1 PN factor also occurred in 1 /1 000 and 1 /1 0000 hyphal suspension treatments. We can conclude from these that EP-1 PN can efficiently transduct its hypovirulence to Ep-1 PNA5 and the transmission efficiency is density-dependent. In the second experiment, the control lesion exceeded 10 cm in diameter after 120 h . When EP-1 PNA5 was challenge inoculated 0, 1, 2, 3, 4, 5,and 6 days after spraying Ep-1 PN hyphal fragements, the lesions were 2.13, 2.80, 3.26, 2.95, 3.95, 3.92, and 3.97 cm in diameter after 120h. So, EP-1 PN can persist for at least 6 days on canola leaves where it can transduct EP-1 PNA5 efficiently. From the two experiments, we can come to a conclusion that hypovirulent isolate EP- 1 PN can effectively transinfect virulent isolate EP-1 PNA5, resulting in EP-1 PNA5 losing its vigorous pathogenicity. It has great potential to control sclerotinia diseases. Field trials are needed to evaluate its potential as a biocontrol agent.

1. Boland GJ, 1992. Can.J.Plant Pathol.43,21-32
2. Li G, Wang D, Huang HC, and Zhou Q, 1996. Acta Phytopathologica Sinica 26,237-242.