Etiology Of Rhizoctonia Head Rot On Brassica Oleracea

The head rot was one of important diseases on cabbage(Brassica oleracea),could cause the whole head rot,and affect the quality and quantity of cabbage.Because of small epidemical area and little economic loss,scientists did not take attention on this disease. There were just a few reports about the disease in the world.In recent years,the head rot of cabbage was observed frequently in Changyang county in Hubei Province,China,and caused tremendous economic loss.The aim of the research was to indentify the caused agent of cabbage head rot,to determine host range and biological characteristics of the pathogen,to know the infection process of the pathogen on cabbage and the change of the cell-wall degradation enzymes in this process.The results showed as follow:The isolate from diseased cabbage could occupy the whole 9mm petri dish within 3d. The fresh mycelia were colorless and then became to brown colored.The diameter of hypha was 4.90-7.98μm.The hyphae branched at right angles or acute angles and had a crinkle at the branch site with a septum around it.Each hypha cell usually possessed more than three nuclei.Sclerotia was irregular,light to dark brown,and had no differentiation in rind and interior.After inoculating the detached cabbage leaves with mycelium plugs of the isolate,the result indicated that R.solani was the pathogen of cabbage head rot. The DNA sequence of ribosomal ITS of the isolate was found to match 99%with an ITS sequence of Thanatephorus cucumeris and R.solani in GenBank,and the information showed both of them were AG 2-1.According to morphological characteristics and analysis of rDNA-ITS sequence,the pathogen was identified as R.solani AG 2-1.The studies on biological characteristics revealed that the optimal conditions for mycelium growth were continuous illumination and alternant illumination.Darkness could facilitate the formation of sclerotia.The mycelium could grow under the temperature from 5 to 30℃,the optimal temperature was 25℃.pH value range for mycelium growth were 4-10 and pH6-7 were optimal.The isolate could utilize various carbon and nitrogen sources,starch and yeast extract were best for mycelial growth.The lethal temperature for mycelial and sclerotia growth were 45℃for 10 min,50℃for 10 min,respectively. The result of determining of the toxicity of fungicides to R.solani AG 2-1 showed that the six tested fundicides could inhibit mycelial growth at different concentrations. 30%Armure EC and 50%Carbendazim WP showed best inhibit to mycelial growth. Whereas the 10%Score GR was the worst.The pathogen could infect 5 cruciferae plants,and Pisum sativum,Vicia faba, Lycopersicon esculentum,Cucumis sativus,Gossypium hirsutum,Zea mays.The cultivars Liangguan,Yongming and Guanjun had some resistance to R.solani AG 2-1,while the cultivars 12089 and 7001 were sensitive to the pathogen.After inoculating the cabbage leaves with mycelium plugs of R.solani,samples were collected from inoculated leaves at different timing intervals,and then decolorated and stained.The result of observing the infection process indicated that the hyphae began to infect the host tissue at 9hai(hours after inoculation).The pathogen R.solani AG 2-1 was able to penetrate host through two pathway such as leaf stoma and epidermis.The stoma infection rate ascended as time goes on.The pathogen could infect directly,and also fromat bulbous appressoria,lobate appressoria and infection cushion before infection. The most mesophyll cells of the host were degradation at 24hai.The results of mensurating cell-wall degradation enzymes at interval times after inoculation showed that the 4 cell-wall degradation enzymes(Cx,PG,PE,PL) were enhanced compared with CK.The activity of Cx and PG had been increased by 100 percent compared with CK at 24hai.All the enzymes reached the maximal activity from 36hai to 48hai,and then descended.The maximal activity of the Cx and PG was 2.64, 2.62 times of CK,respectively,and the PE and PL just 1.65,1.59 times.After mensurating cell-wall degradation enzymes in the filemot area and black area of the lesion 96hai,the activity of Cx and PG were significantly higher than PE and PL in both of filemot and black area.The enzymes activity of filemot area was the highest,and the activity of Cx and PG was 3.18,3.39 times of CK respectively,and was 2.12,2.28 times of the black area.In conclusion Cx and PG were the most important factors in the process of R.solani AG 2-1 infection and host cells breakage.