Study On Infective Character And Correlative Factor Of Sweet Potato Stem Nematode Disease

1. The infection of Ditylenchus destructor in sweet potatoThe penetration of Ditylenchus destructor in sweet potato was studied in natural field condition with an inoculation method. The results showed that the nematodes penetrated to the seedling from its root tip (3 cm deep from soil surface) and then migrated to upside in the seedling. The nematodes were not extracted from the seedlings after 14d transplanting.The nematodes could migrate in underground stem close to ground after 28d. The nematodes reached to the aerial stem of a plant and caused damage after 56d. The nematodes migrated to the new tuber roots and caused the damage after 84d, but the nematodes were not extracted from the fiber roots.2. The population dynamics of Ditylenchus destructor in sweet potatoThe population of Ditylenchus destructor in sweet potato was studied in natural field condition with an inoculation method. The disease incidence was 40% and disease indexe was 10 in the treatments of 1-2 nematodes per gram soil after 24d transplanting, and disease incidence reached to 100% after 60, the disease indexe was 52-60 in 120, while the disease incidence was 60%-80% and disease indexe was 20 in the treatments of 16-64 nematodes per gram soil in 24d, and the disease index reached as high as 60-100 after 120. The disease incidence reached 100% only in 24d and disease index was 100 after 96 in the 256 nematodes per gram soil. The numbers of nematode in a stem of sweet potato were 49-56 in the treatments of 1-2 nematodes per gram soil after 60d, while its numbers were about 200-300 in the treatment of 16 nematodes . The highest number of nematode in a seedlings was about 60,000 in the treatments of 64-256 per gram soil after 96d,since then the numbers of nematode started to decline. The results determined the nematodes penetration site D. destructor to the sweet potato seedling and the linear relation between the numbers of nematode in the soil and disease index.3. Complex infection caused by fungi and Ditylenchus destructor to sweet potatoComplex infection disease caused by Ditylenchus destructor and 7 kinds of fungi including Fusarium oxysporum, Fusarium solani, Fusarium graminearum, Ceratocystis fimbriata, Rhizopus stolonifer, Mucor javanicus, Penicillium terrestre respectively were studied on natural field condition with an inoculation method. The results showed that co-infection of D. destructor with F.oxysporum, F.solani, C.fimbriata and R.stolonifer could caused complex disease of sweet potato which aggravated disease.The disease incidence and disease index as well as the numbers of nematode in the sweet potato were significantly higher than those in nematode infection alone. No significant difference found between inoculating F.graminearum or M. javanicus with D. destructor together and inoculating D. destructor alone. However the nematode population was inhibited in the co-infection of D. destructor and P. terrestre which alleviated disease.4. Chemotaxis of Ditylenchus destructor in response to different inorganic ionsThe chemotactic responses of Ditylenchus destructor was studied in salt gradients created in an agarose gel. Nineteen combinations of sodium, calcium, potassium, ferric or ammonium cations and chloride, nitrate, sulphate, hydrogen-phosphate, bicarbonate and carbonate anions were tested at six concentrations from 0.0625×10-2 to 2×10-2 mol L-1. Ditylenchus destructor was attracted to salts that included Cl-, and NO3-, whereas salts with SO42- and H2PO4- anions were repellent, but the salts comprising CO32- had almost no effect on the nematode movement. The repellent or attraction properties of different salts having the same cations were not consistent. The chemotaxis of nematodes to Ca(H2PO4)2, KH2PO4, NH4H2PO4, K2CO3, FeSO4, Na2SO4, K2SO4, CaSO4, and KNO3 increased with increasing concentration, whilst the concentration of other salts tested did not influence nematode chemotaxis significantly.5. Effect of several nematicides on the population dynamics of Ditylenchus destructorThe effect of 4 kinds of nematicide on the population dynamics of Ditylenchus destructor was conducted on natural field condition with an inoculation method. The results showed as follows:(1) Sweet potato could be effectively protected from nematode infection by 4 kinds of nematicides at different concentration including 1.8%avemectin,20%phoxim ,25% chlorpyrifos and 20% carbosulfan EC. The high concentrations were more effective than the low ones.After 80 days,both of avemectin 180μga.i.mL-1 and carbosulfan 2,000μga.i.mL-1 had the best protection effect. The disease index was 32 and the number of nematode in a seedling was 145,while the control was over 80 and 4,689 respectively. The number of nematode in a seedling was increased at different treatments with increasing the cultural time.(2) Sweet potato stem nematode disease was cured by above 4 species of nematicides at 3 different concentrations in different times.Compared to the control, the effect of therapy of each nematicide with high concentration was more effective than the low one. Both of the disease index and the number of nematode in the plant were rather low. 45 days later, the avemectin 180μga.i.mL-1 and carbosulfan 2,000μga.i.mL-1 had the best remedial effect also. The disease index was 24 and 32 respectively and the number of nematode in a seedling was low 70. However the disease index and the number of nematode of the control were 72 and 2,336 respectively.(3)The period of validity for Ditylenchus destructor to chlorpyrifos 2,500μga.i.mL-1and phoxim 2,000μga.i.mL-1 were 30 and 40 days.The number of nematode in a treated plant was lower than the control and significant difference found between them.However the effect of carbosulfan 2,000μga.i.mL-1and avemectin 180μga.i.mL-1 were at least 50 days.