Identification Of Plant-parasitic Nematodes Associated With Citrus Plants And Screening Of Control Chemicals In Guangdong

Guangdong Province is a large production area of citrus plants in China,where development of related industry is strong,and is largely challenged by nematode as an important pathogen of the plants.Nematode has become a limiting factor affecting the yield and quality of the citrus plants,and often severely affecting the growth of the plants,resulting in growth weakening and shortening of the plants,yellowing of the leaves and eventually dying of the plants.Thus,to make clear related species of plant-parasitic nematodes is of significance for effective control of nematode diseases and then keeping of healthy growth of the citrus plants.However,little systematic information on diversity and control of nematode species from the citrus plants in Guangdong Province is known.Therefore,in the present study,total 96 soil samples around the rhizosphere of the citrus plants in 16 citrus planting areas in Guangdong Province were collected.Plant-parasitic nematode species were identified according to morphological and molecular characterization.At the same time,phylogenetic relationships among the nematode species based on sequences of rDNA small subunit（SSU）18S,the D2-D3 fragment of large subunit（LSU）and ITS were analyzed,and the relevant chemicals to control Meloidogyne sp.were screened in the laboratory.The main results were as below:1.Based on morphological and molecular characteristics of nematodes,10species of plant-parasitic nematodes were identified,including Aphelenchus avenae,Helicotylenchus crenacauda,H.dihystera,Hemicriconemoides strictathecatus,Mesocriconema xenoplax,Pratylenchus coffeae,Rotylenchulus reniformis,Tylenchorynchus leviterminalis,Tylenchulus semipenetrans,Xiphinema hunaniense,and other two undetermined species of the nematodes,i.e.,Aphelenchoides sp.and Meloidogyne sp.were also identified only based on the morphological characters.2.These studied 12 species of parasitic nematodes belong to two orders,nine families and ten genera.Among them,there were four dominant nematodes,i.e.,A.avenae,H.dihystera,M.xenoplax and R.reniformis;five new records of genera of parasitic nematodes of citrus plants in Guangdong Province,i.e.,Aphelenchoides,Aphelenchus,Hemicriconemoides,Mesocriconema and Rotylenchulus;three new records of species of parasitic nematodes of citrus plants in China,namely,H.strictathecatus,T.leviterminalis and X.hunaniense;and six new records of species of parasitic nematodes of citrus plants in Guangdong Province,including A.avenae,H.crenacauda,H.dihystera,H.strictathecatus,M.xenoplax and R.reniformis.3.In China,lianjiang orange is potential new host for six nematodes,i.e.,A.avenae,H.crenacauda,H.dihystera,H.strictathecatus,M.xenoplax and T.semipenetrans;lemon for five nematodes,i.e.,A.avenae,H.dihystera,M.xenoplax,R.reniformis and T.semipenetrans;bergamot for five nematodes,i.e.,A.avenae,H.strictathecatus,M.xenoplax,T.leviterminalis and X.hunaniense;shatian pomelo for four nematodes,i.e.,A.avenae,H.dihystera,H.strictathecatus and R.reniformis;Citrus reticulata cv.Chachiensis for four nematodes,i.e.,A.avenae,H.dihystera,R.reniformis and T.semipenetrans;sweet orange for four nematodes,i.e.,Aphelenchoides sp.,H.dihystera,R.reniformis and T.leviterminalis;and kumquat for four nematodes,i.e.,A.avenae,P.coffeae,T.leviterminalis and T.semipenetrans.4.The differences in the distribution for 12 parasitic nematodes from different citrus planting areas were presented.Among them,H.dihystera and A.avenae were most commonly distributed in the sampling places,the detection rates were 81.3%and 75.0%,respectively,followed by M.xenoplax and R.reniformis,both detection rates were 62.5%,and are again T.leviterminalis,T.semipenetrans and H.strictatectus,the detection rates were 50.0%,50.0%and 43.8%,respectively.5.Molecular phylogenetic analysis indicated that the studied 10 parasitic nematodes associated with citrus plants were respectively located at reasonable position in phylogenetic trees inferred from either SSU 18S or LSU D2-D3 fragment sequences,and they were grouped in a mono phylogenetic clade with their respective same populations with high support（posterior probability）,which confirms the results of identification based on morphology.6.In the laboratory,abamectin 1.8%EC and thiazolifos 10%EC had good control effect on the eggs and the juveniles of Meloidogyne sp.The inhibition rates of them to eggs hatching were 99.56%and 98.89%,respectively,and the virulence LC₅₀values to the juveniles were 15.4704 and 22.2698 mg/L,respectively.