Resistance evaluation of different mango cultivars to Oligonychus mangiferus (Rahman et Runjab)

Abstract:【Objective】Oligonychus mangiferus is one of the most destructive pests in mango trees. It feedsprimarily on the lower surfaces of the leaves. During feeding, the mites penetrate the host plant with theirstyles and suck out the cell contents, resulting in discoloration and death of the leaf tissues. Heavy infesta⁃tions result in severe defoliation, causing mango trees to stop growing and also results in fruit yield reduc⁃tion. If young mango trees have been seriously damaged, their twigs will eventually become dry and die. Inrecent years, O. mangiferus has become one of the most destructive pests throughout most of the mangotree plant areas in China. For a long time, the control of this mite was mainly dependent on the chemicalacaricide, however, the question of environmental pollution and“3R”has also become more and more aserious concern. Host plant resistance has been considered as the most active, effective and economicmeasures against insects and mites, and has already become a directional measure for pest controls bothat home and abroad. The planting of a variety of resistances to mites were the most effective and direct ap⁃proach to control the spider mite, in view of the fact that there’s no definitive mite-resistant mango germ⁃plasms at home and abroad so far, and also because of the lack of a systematic research theory and technical support of mite-resistant mango germplasms. Furthermore, according to the development and the actu⁃al needs of the mango industry, after establishing the mite-resistant evaluation criteria of rubber germplasms and obtaining the stable resistant and susceptible mango germplasms, this study compared the dif⁃ferences of the survival of O. mangiferus feeding on resistant and susceptible mango tree leaves and thecontents of nutrients, secondary metabolites products.【Methods】Cultivars were evaluated for resistanceat sites in which they were growing vigorously and in fields with the same ecological characteristics. Theevaluations were facilitated by natural infestations of O. mangiferus. Ratings were based on the followingdamage scale: 0. No mites or symptoms; 1. a few mites on leaves, some yellow to white speckling onleaves; 2. a large amount of mites on leaves, red speckling on leaves; 3. leaves full of mites, the area percentage of the red speckling on leaves is between 1/3 to 2/3; 4. lots of mites on the entire plant, the areapercentage of the red speckling on leaves is beyond 2/3. Since no definitive mite-resistant mango germ⁃plasms against O. mangiferus exist, this study evaluated the mite resistant cultivars via the rate of dispersion. First, the average values of the index of mite damage (I*) for all the evaluated cultivars and the highest score of index of mite damage (I) for each cultivar in three replicates were obtained, then we obtainedthe degree of resistance via I/I*. After obtaining the cultivars that steadily developed resistant to O. mangiferus via the field and laboratory, we compared the differences of the contents of free amino acids, freesugar, nitrogen and the ratio of sugar and nitrogen in different mango cultivar leaves. The contents of freeamino acids, the free sugar, the free nitrogen as well as the total phenol were measured by the ninhydrincolorimetry method, anthrone method, the Coomassie brilliant blue G250 staining method and the catechins colorimetric method, respectively.【Results】O. mangiferus only damaged the functional leaves butnot the tender and the older leaves of the mangos. Two mango cultivars steadily developed resistance to O.mangiferus,‘Tainong I’and‘Hongmang VI’, and four susceptible cultivars,‘Jidanmang’,‘Qiumang’,‘Zihua’and‘Yuexi I’, were screened from 21 mango core cultivars on the basis of the established miteresistantevaluation criteria of mango germplasms.‘Tainong I’and‘Hongmang VI’were highly resistantand resistant respectively, while‘Jidanmang’,‘Qiumang’,‘Zihua’and‘Yuexi I’were susceptible to O.mangiferus. Significantly different survivals of O. mangiferus were observed among different mango culti⁃vars. The survival rate of O. mangiferus fed on functional leaves of‘Tainong I’and‘Hongmang VI’were5.17% and 32.33%, respectively, significantly lower than those of the other 4 cultivars (58.16% for‘Jidanmang’, 62.27% for‘Qiumang’, 77.36% for‘Zihua’and 76.67%‘Yuexi I’). The resistance of the eightcultivars against O. mangiferus were‘Tainong I’＞‘Hongmang VI’＞‘Jidanmang’＞‘Qiumang’＞‘Zihua’＞‘Yuexi I’in the field, and were accordant with those in the laboratory. Significant differencesfor the contents of nutrients and the secondary metabolite products in different mango cultivars were ob⁃served and require further comparison. The nutrient contents in the mite resistant mango cultivar leaveswere significantly different with those in the susceptible rubber germplasm leaves (P< 0.05). There weremore free sugar, high ratio of sugar and nitrogen and more total phenols in the functional leaves of the resistant cultivars‘Tainong I’and‘Hongmang VI’than those in the functional leaves of the susceptiblecultivars‘Zihua’and‘Yuexi I’, which were significantly different between the resistant and the susceptible cultivars; contents of free sugar, ratio of sugar and nitrogen and contents of total phenols in the tenderand the older leaves and contents of free amino acids and free nitrogen were not significantly different between the resistant and the susceptible cultivars.【Conclusion】The above results all indicate that the contents of free sugar, ratio of sugar and nitrogen and total phenols in the functional leaves were significantlyrelative to the resistance of mango cultivars to O. mangiferus. We can therefore elucidate the mite-resistant mechanism of the mango germplasms, and at the same time, provide basic information, experimentalmaterial, and theory support for the molecular design breeding in mite-resistant mango germplasms.