Relationship between the leaf structure and its resistance to powdery mildew in pear

Abstract:【Objective】Pear is one of the main fruits in the world. It is the third largest fruit after apple and citrus in China, and its output and area rank first in the world. However, the pear producing areas in China are often affected by many diseases, among which powdery mildew caused by Phyllactinia pyriis one of the most common and serious diseases in the northern pear producing areas. This disease mainly harms the leaves and causes early fallen leaves, which resultes in the reduction of the fruit quality, and also decreases production and leads to serious economic losses. Therefore, this study evaluated and clarified the resistant levels of different pear varieties, and discussed the relationship between pear leaf structure and its resistance to P. pyri.【Methods】A total of 8 varieties, including Huangguan, Yuluxiang, 9-31, Ganli No.3, Zaosu, Xueqing, Xinli No.7 and Qiuyue were collected from Tiaoshan Group, Jingtai County, Baiyin City, Gansu province. The experimental management was extensive, and powdery mil- dew disease was common and serious year by year. The incidence of powdery mildew caused by P. pyriwas investigated with different varieties, when the leaves were collected in the field during 2018 to 2020. Three trees were investigated for each variety, and each tree was investigated with two new termi- nal shoots from east, west, south, north and middle of the canopy, five directions. Each new tip was in- vestigated on 10-15 leaves from top to bottom and the number of diseased leaves at all levels were recorded, and the disease indexes were calculated according to the results obtained. The leaf damage level could be divided into 0, 1, 3, 5, 7, 9, corresponding to 0, 0-5%, 6%-25%, 26%-50%, 51%-75%, and over 76% of the total leaf area, respectively. According to the disease indexes (DI) of different pear vari- eties to P. pyri, DI 0 was immunity (I), DI≤5 was high resistance (HR), DI 5-25 was resistance (R), DI 26-50 was susceptible (S) and DI 51-100 was high susceptible (HS). The stoma densities after the epi- dermis were tore off by tweezers were measured under the microscope with one vision as the detection unit, examining fifty visions for each variety. The numbers of stomata on the back leaves were observed and counted in different parts, and the average value was calculated. According to the hair density, it was divided into 5 levels under 100 times magnification under a laboratory microscope, among which level 1 was unhairy and smooth, level 2 was unhairy and rough, level 3 was sparse-hairy, level 4 was medium-hairy and level 5 was dense-hairy. The leaf anatomy was observed by paraffin dissection. Leaf thickness, upper or lower epidermis thickness, and fence or sponge tissue thickness were measured and recorded by the leaf cross section. After measuring the leaf area with a Ci-203 meter, the leaves were dried thoroughly in the oven and the dry leaf weight was measured. Each sample repeated 50 times and took the average value. All data were calculated by Excel processing. Multiple comparisons and vari- ance analysis were analyzed by using DPS 16.0 statistical software. Relevant analysis between leaf structure index and varieties resistance was analyzed with SPSS 22.0 software.【Results】The resistant level of different varieties to P. pyri was significantly different. The incidence and disease index of Huangguan, Yuluxiang, Ganli No.3 and 9-31 were 0 to demonstrate immunity. Zaosu had the highest index 86, then Xueqing reached 80, and the lowest disease index was over 70. The leaf wax content was significantly different among pear varieties. Resistant varieties were generally higher than susceptible ones.Thehighestwaxcontentreached 9.56mg·g-1 andthelowestwas3.3mg·g-1. Asignificantnega- tive correlation was found between the leaf wax content and the disease index, and the correlation coef- ficient was -0.735. The varieties with high wax content had strong resistance to P. pyri. The leaf weight with different resistance was not obvious and was not related to the resistance. The stoma density of varieties was different and had no relation to P. pyri. Hair density was significantly positively associated with a coefficient of 0.909, which demonstrated the less hair density, and more resistance. Ganli No.3 had the most thickness and reached 537.13 μm, followed by Xueqing 313.48 μm, and the thinnest was Yuluxiang 203.08 μm. The leaf thickness was not related to P. pyri. The upper epidermis thickness of different varieties was higher than the lower epidermis, and the difference was not obvious. Ganli No. 3 and Xueqing had the thickest fence and sponge tissue, which reached 129.64 μm and 112.15μm, respec- tively, then Xinli No.7 was 117.31 μm, and the thinnest varieties were Huangguan and Yuluxiang, which reached 90.5 μm and 571.1μm, respectively. The thickness of fence or sponge tissue of different varieties was not related to their resistance. The compact structure of different varieties was higher than that of loosen structure. The loosen structure was not different among varieties, and had no relation with the resistance to P. pyri.【Conclusion】The significant difference among varieties and their resistance toP. pyri existed. Stoma density, leaf weight ratio, leaf thickness, upper epidermis thickness, lower epider- mis thickness, fence or sponge tissue thickness had no significant correlations with the resistance toP. pyri. However, there were significant correlations between hair density and wax content. Therefore, leaf wax content and hair density could be used as main reference indexes of the pear’s resistance to P. pyri.