Identification And Control Of Alternaria Sp. In Citrus Orchards

Citrus is the most important fruit in the world, China is one of the most important centers of origins, its cultivation area and annual production are at the top of the world. Alternaria sp. an important pathogen of plants, has variety and wide host range, the adaptability to environment and matrix in nature is very strong, widely distributing in environment. It is also important in citrus orchards, infects the leaf,stem and fruits of the citrus, causing Allbinism albino, rough lemon Alternaria leaf spot and Alternaria stem-end rot, Alternaria brown spot and Alternaria black rot,which leads to the huge economic loss.Recently, Alternaria sp. infected leaves and twigs of citrange seedling, and a large number of seedings dead suddenly in the southwestern region of Chinese citrus orchards, it was also reported that Alternaria sp. is one of pathogens of Kumqua. Study on the pathogenic mechanism of the Alternaria sp., and screening the fungicides to Alternaria sp. under laboratory condition, which lay the foundation to control the pathogen. The paper studied on morphological, genetic diversity of 7 Alternaria sp., conducted a preliminary study of the pathogenic mechanism of Alternaria sp. in citrus orchards, made the poison power test of 20 fungicides. The main results were obtained as followed:1.7 Alternaria sp. isolated from tissues affected were coded as MP2,MP3,MP4,JG2,JG4,T1-4 and T2-3.The various stains had some differences in morphological characteristics by observed with microscopic observation and detection of pathogenicity.The strains were identified as Alternaria sp.only.2. The cluster analysis of the rDNA-ITS sequences showed that all strains were separated to two branches. MP2 and MP4 clustered into a single with Alternaria tenui,Alternaria astragali, Alternaria sp. OUCMBI101196.MP3, JG4, T1-4, T2-3 clustered together with Alternaria alternate, Alternaria citri, Alternaria longipes. The result of RAPD analyzed that all strains were separated to three branches, MP2, MP4 and MP3 were clustered into a single branch. JG4,T1-4 and T2-3 were clustered into one branch.JG2 was the other one. Results of rDNA-ITS and RAPD indicated that MP2 and MP4 were same species or similar species, they had closely relationship with A.tenui,A.astragali,Alternaria sp. OUCMB1101196. MP3, JG4, T1-4, T2-3 and A.alternate, A.citri, A.longipes had high homology, they were similar species. But JG2 was needed to identify further. It lay the foundation to classification and fast identification of Alternaria sp..3. It is observed through color reaction that the structure of MP3 toxin is ketone, which is similar with ACT toxin produced by A.citri. The result is that the activity of POD and APX in cells of leaf treated with MP3 toxin declined, and the content of MDA rose. It showed that MP3 toxin inhibited function of enzyme system, cause scavenging system of activated oxygen damaged, resulted active oxygen accumulated, which led to oxidizing reaction of plasma membrane, producing MDA, of which the content increased with concentration of MP3 toxin increasing.It proved that the site with MP3 toxic played the role is plasma membrane, which is same to ACT toxin.4.20 fungicides to Alternaria sp. were screened the under laboratory condition. The result showed the degrees inhibition of different fungicides has distinction to affect the mycelium of MP3 with different densities, and the inhibition rate was positively correlated with the concentration.The EC5o of 70% Propineb was minimu, the inhibitory effect of the best, followed by 10% Dife-noconazole WP,50% Propiconazole, 27.12% Magnesium sulfate, 16% of the resin Copper yoghurt, 10% Jinggangmycin, 400g/L DuPont lucky star,70%Copper hydroxide,22% Hexacon-axole,1.5% Thiophene mold copper, 65% Mancozeb.So far, the best fungicides are 65% Mancozeb and Copper fungicides.It offered a reference to control the Alternaria sp..