38 Phytophthora parasitica var.nicotianae isolated from different places in Guizhou Province were analyzed for genetic diversity by using RAPD,and genetic distance and relationship of accessions were discussed.The results showed that 65 DNA bands were found from 38 strains by PCR and 59 bands accounting for 90.7% of them were polymorphic,which indicated that there was abundant genetic diversity in tobacco in Guizhou province with different genetic structures among tobacco collected from different area.According to fingerprint spectra,all of 38 strains were divided into nine genotypes by UPGMA with genetic similarity coefficient of 0.74,which showed no distinct correlation between geography distribution and strain genotype.

Different base medium adding different combination of antibiotics were applied for isolation and culture of Phytophthora nicotianae.The results showed that selective medium with V8 base medium(100 mL/L V8juice+0.02 g/L CaCO3+2 g/L agar) adding 10 mg/mL ampicillin +5 mg/mL nysfungin+5 mg/mL carbendazim had the best isolation effect as the successful isolation rate was 100%,and the hypha grew well on this medium.After separation,the hypha could be cultured in V8 base medium as the hypha was white,tight and strong with big bacteria clone and strong growth potential.

To confirm the sensitivity of Phytophthora nicotianae var.nicotianae to dimethomorph,forty seven isolates of P.nicotianae var.nicotianae were collected from several areas in Guizhou Province,and sensitivity of mycelial growth and zoosporangia formation to dimethomorph were measured.The results showed that dimethomorph could inhibit the mycelial growth and sporangium formation of P.nicotianae var.nicotianae,for mycelial growth,with a mean EC50 value of 2.476 8 μg/mL,ranging from 0.792 6 μg/mL to 4.232 7 μg/mL;and for sporangium formation,with a mean EC50 value of 2.040 0 μg/mL,ranging from 0.394 5 μg/mL to 4.232 7 μg/mL.Sensitivity of these isolates to dimethomorph was fitted well with a normal distribution function.There were significant differences in the sensitivity of different strains to dimethomorph,and moreover there was a correlation between strain sensitivity and its geographical origin.

In recent years Fusarium wilt had a wide distribution and severe damage to P.capitatum in Shibing County in Guizhou,which had already been the potential threaten to local P.capitatum production.In this paper the pathogen in typical diseased tissue of P.capitatum was isolated and identified and a tie-back test was conducted to control the occurance and damage.The results showed that the three isolated pathogens were Cochliobolus curvularia,Rhizoctonia solani and Fusariumch lamydosporum.The inoculation test showed that only F.lamydosporum could cause symptom in accordance with field disease.After morphological identification and ITS sequence analysis,it was preliminarily concluded that F.lamydosporum was the pathogenic bacterium.Bacteriostasis test showed that prochloraz,carbendazim,chlorothalonil and hymexazol had preferable inhibitory action on F.lamydosporum.

The inhibitory effect of the four Chinese medicinal herb extract,which include Sophora flavescens(A),Lonicera japonica(B),Scutellaria chihshuiensis(E) and Psoralea corylifolia(F),against Phytophthora var.nicotianae were assayed,by applying Hypha Growth rate method.Then toxicity regression equation was obtained.EC50 value of four herbal extracts were 4.56、5.99、2.65、4.76 mL/L respectively.The synergistic effect of pairwise compounds of the four herbal extracts was determined according to the method of co-toxicity factor.The result showed that compounds of A and B,A and F,B and F,have obvious antibacterial effect.It finds that the inhibitory effect of the compounds of the four herbal extracts against Phytophthora var.nicotianae was synergitic.The inhibitory effect of the compounds was superior to that of simple dosage.And the best ratio of compounds was screened.