The Relation between the anatomical characters of five different cucumber cultivars leaves and Tetranychus urticae koch ( acarina : actenididea : tetranychidae : ) infestation under filed conditions

The relative population densities of Tetranychus urticae Koch (Acarina: Actenidida : Tetranychidae) movable stages infesting leaves of five cucumber cultivars (Impress F1, Yaaqube, Prince 3, Madina 2 and Sahm) during 2009 and 2010 seasons was investigated at Qalyubia Governorate. The results indicated significant differences between the tested cultivars in their relative susceptibility to T. urticae infestation. In the two tested seasons the five cucumber cultivars were grouped into four groups, as the highly infested cultivars were Impress F1 and Yaaqybe while the lowest infested group included Sahm cultivar, which was characterized by somewhat the thickest layer of the upper & lower epidermis and palisade tissue and the thinnest layer of spongy tissues. While, leaves of Impress manifested approximately the thinnest layers of the upper & lower epidermis and palisade tissue) and the largest diameter of Spongy tissue. Insignificant negative correlation coefficient values were calculated between the mean abundance of T. urticae individuals infesting different cultivars and the relation of thickness of the upper & lower epidermis, and palisade tissues while the relation with the spongy layers was positive i.e., cultivars characterized by thicker layers of the previously mentioned three layers were less susceptible to this pest infestation and subsequently, infested by fewer population of the two spotted spider mite and vice versa.


INTRODUCTION
Leaf structure has been showed to be related to mite damage or to symptoms commonly associated with damage.The highest density of phytophagus mites occurred on leaves which, in addition to having higher total nitrogen content, had a thicker palisade mesophyll.In this connection, size of the leaf is also important.So that the leaf surface texture, and contours (ridges, depressions) are important in the reproductive potential of a resident phytophagus mites has long been recognized.Many studies were done to clarify the relation between mite infestation and leaf structure.Some varieties of Pelargonium with thicker cuticle and epidermis were more resistant to spider mite.Besides that, the upper leaf surface was less preferred by mites, because its cuticle and epidermis were, thicker than that of the lower surface (Kou et al., 1972 andLuczynski et al. 1990).Kielkiewicz, 1994 andMagali, 1997 stated that, leaf trichomes contributed to the reduction of T. cinnabrinus density at beginning of mite feeding and on tomato and bean varieties.Adults of T. urticae could feed through the spongy and part of the palisade parenchyma of the leaf, while immature T. urticae could feed only through the spongy parenchyma (Park and Lee, 2002).Also, the length, density and thickness of leaf trichomes may be considered as anther factors affecting the host plant resistance to infestation by T. urticae (El-Saiedy Aziza M. M. Abou-Zaid 28 2003).A negative relationship between the thickness of the upper epidermis and spongy tissue of leaf of the 7 cucumber varieties and the abundance of T. urticae movable stages.While, it was positive in case of palisade tissue (Hanafy, 2004).Moreover, T. urticae preferred the lower leaf surface of the plants due to the very thin cuticular layer, (Abo- Bakr and Ali, 2005).Also, the more thickness of the cuticle of the epidermis especially that of the lower surface could be considered as a physical resistance factor (Azouz, 2005).Resistant varieties possess thicker on either upper or lower leaf surface than susceptible ones.The more thickness of the cuticle epidermis especially that the lower surface could be considered as a physical tolerance factor against mechanism of spider mit (El-Sanady et al., 2008).
The present work aims to evaluate the relative susceptibility of five cucumber cultivars to the spider mite, T. urticae infestation with relation to the anatomical characters and mite level infestation.

MATERIALS AND METHODS
For the evaluation of the susceptibility of some cucumber cultivars the twospotted spider mite, T. urticae infestation, five different cucumber cultivars (Impress F1, Yaaqube, Prince 3, Madina 2 and Sahm) were cultivated in an area of about 175 m 2 in Qalyubiya Governorate during two successive seasons 2009 and 2010.The seeds were sown in hills every season on 15 th April.The experimental area was divided into 15 plots, each plot area was about 50m 2 .Each tested cultivar was represented by three replicates which arranged in a complete randomized block design.
All the recommended agricultural practices were carried out as the need of the cultivated plants.Also, the experimental area was kept free from any pesticide treatments.On the other direction, the plants were left to the natural infestation and no artificial infestation was conducted.
To clarify the relative susceptibility of five cucumber varieties to T. urticae infestation in relation the anatomical characters of their leaves, samples of each variety were taken during 2009 and 2010 growing seasons at (seedling and flowering + fruiting) stages and placed in the fixed solution FAA four 48 hours then transferred to 70% alcohol.Then after, a transverse sections in there leaves were made by using the method described by Jackson (1976).The different measurements (in micron) of the thickness of upper epidermis layer, palisade tissue, spongy tissue and lower epidermis layer were determined by using of Compu Eye, Leaf &Symptom Area program by Bakr (2005).

Sampling technique:
Three weeks after sowing of the open field cucumber seeds, ten randomly chosen leaves from different levels of plants were picked up from each replicate and then kept in tightly closed paper bags and transferred to the laboratory at the same day where the number of eggs, immature and adult stages of T. urticae were estimated by counting the total number per two randomize chosen square inches on the low surface of the leaves with the aid of a stereomicroscope.Samples were taken weekly, 7 days intervals, for each experiment.Then the sampling continued for 12 weeks.

Statistical analysis:
The statistical analysis (ANOVA and Simple correlation) of the obtained data were performed by using SAS program (SAS Institute, 1988) which run under WIN.Also the difference between means was conducted by using Duncan's multiple range tests in this program.
The relation between the anatomical characters of five different cucumber and T. urticae 29
Concerning the statistical analysis of the obtained data by using the least significant differences (L.S.D.) between the previously mentioned means, the results indicated that there are significant differences between the tested cultivars in their infestation by T. urticae.For the first season 2009, L.S.D. and F values were 2.46 and 9.3, respectively, clearing that the five cucumber cultivars were grouped into four groups as following: a-the highly infested; Impress F1 and Yaaqube cultivars, b -the moderately harbored ones; represented by 50.55 cultivar, while Madina 2 cultivar, represented by (c) group and d-the lowest infested group included Sahm cultivar.In the subsequent season 2010, the same four categories were observed (L.S.D = 17.93&F= 10.03) but the significant difference between the means were less as there were no significant difference between Impress F1 and Yaaqube cultivars as they were classified as the most susceptible to T. urticae infestation (a group) followed by Prince 3 and Madina 2 cultivars (moderately infested category, bc and cd), comes next Sahm cultivar which was more resistance to T. urticae infestation (d group).(

Relation between anatomical characters of leaves from different cucumber cultivars and the two-spotted spider mite infestation:
Thickness of leaf is a character that contributes towards resistance and hence effects the mite pest population.So the susceptibility of the five tested cucumber varieties (Impress F1, Yaaqube, Prince 3, Madina 2, and Sahm) to the infestation with moving stages of red-spider mite was related to the anatomical characters of leaves of each variety, such as the thickness of the leaf layers (upper epidermis, palisade, spongy tissues and lower epidermis).This part of study was done to discover the correlation between these layers and the mean population density of movable stages of T. urticae during two growth stages during the two studied seasons, seedling stage in May, 26 th and flowering + fruiting in July, 21 st .Tables (3&4) give both of the means count on each variety and mean thickness of each of the 4 tested layers and the correlation coefficient values between these tested factors.
The lowest determination of T. urticae' infestation (25.08 individuals / inch2) were observed on Sahm cultivar.Such cultivar was characterized by somewhat the thickest layer of the upper & lower epidermis and palisade tissue (17.48 & 14.98 and 110.67 micron, respectively) and the thinnest layer of spongy tissues (52.56 micron), while leaves of Impress F1 which harbored the highest number of T. urticae manifested approximately the thinnest layers (12.12, 89.51 and 10.76 micron for the upper & lower epidermis and palisade tissue) and the largest diameter of Spongy tissue (84.25 microns).The same observations were recorded for Yaaqube cultivars which had no significant difference with Sahm cultivar in its infestation with this pest, as the mean diameters of its leave were 14.44, 89.12, 75.78, and 12.70 µm for upper epidermis, palisade tissue, spongy tissue during 2009, respectively.
Data tabulated in Table (3) showed that, the differences between means of infestation rates by T. tabaci to cucumber leaves of the experimented five cultivars are either significant or insignificant.Insignificant negative correlation coefficient values were calculated between the mean abundance of T. urticae individuals infesting different cultivars in relation to thickness of upper & lower epidermis, and palisade tissues ( r = -0.38,-0.31 & -0.42, respectively) while the relation with the spongy layers was positive (r = 0.13); i.e., cultivars characterized by thicker layers of the The relation between the anatomical characters of five different cucumber and T. urticae 31 previously mentioned three layers were less susceptible to this pest infestation and subsequently, infested by fewer population of the two spotted spider mite and vice versa.The amount of variability that could be attributed to the combined effect of leaf anatomical characters of cultivars on the different stages of T. urticae population on the five tested cucumber cultivars were more than 70% for the first season 2009, as the overall explained variance (E.V.%) was 75.47 (Table 3).
Statistical analysis of data as shown in Table (4) revealed that the seasonal mean rate of infestation by T. urticae moving stages to leaves of 5 cucumber cultivars showed insignificant correlation with the thickness of the upper & lower epidermis, and palisade tissues, beside that, they were also negative (-0.07 & -0.27 and -0.32, respectively), on the other hand, a significantly positive relation with the spongy tissue (r = 0.70).
The negative correlation between seasonal mean population density and upper and lower epidermis layers during 2010 season could be noticed with Impress F1 cultivar which was heavily infested by T. urticae moving stages (72.27individuals/ in 2 ) and manifested the thinnest layer (14.96 and 13.39 micron), while Sahm cultivar which showed lower infestation rates (27.78 individuals / in 2 ) were characterized by thicker upper & lower epidermis layers and Palisade tissue (17.03 & 15.65 and 110.45 micron, receptively).Also, the positive correlation with thickness of the spongy tissue were clear in cases of Impress F1 and Yaaqube cultivars which showed the highest infestation rates (72.27 and 58.40 individuals/ inch 2 ), combined with the spongy tissue thick of 81.70 and 75.07 micron, respectively.In the same direction, Prince 3 showed moderate infestation of the movable stages (44.52 individual/in 2 ) had the moderate diameters of the tested leaf layers (16.19, 13.59, 92.41 and 64.06 µm for upper& lower epidermis, palisade and spongy tissue, respectively.(Table 4).The obtained results revealed that, significant effect for the combined effect of the tested leaf layers on the pest activity during second season 2010, as the amount of variability, attributed to the combined effect of the four layers on the two-spotted mite population on the five cucumber cultivars was 57.22%.
In a conclusion, the result regarding correlation between some anatomical cucumber leaf characters and mite population revealed that most of the combinations were found to be negative.This means that Thickness of leaf is a character that contributes towards resistance and hence effects the mite pest population.As it was obvious from data packed from the previous information that, the maximum thickness of leaf upper and lower epidermis (μm) was recorded on Sahm cultivar which statistically infested with lowest numbers of T. urticae among other tested cultivars, while Impress F1 had the lowest leaf diameters and highest number of individuals.
This means that, leaf thickness is very important factor influencing mite population and showed significantly negative correlation with mite population.The results agree with Saber and Momen, 2005& El-Sanady et al., 2008and Shakoor et al., 2010 who reported that leaf toughness and thickness are very important factors, which affect the reproduction and development of mite population.

Table 1 :
Population fluctuation of the moving stages of Tetranychus urticae / inch 2 , counted on leaves of five cucumber cultivars in the field during 2009 seasons in Qalyubia Governorate Inspections date

Table 2 :
Population fluctuation of the moving stages of Tetranychus urticae / inch 2 , counted on leaves of five cucumber cultivars in the field during 2010 season in Qalyubia Governorate.

Table 3 :
Relation between anatomy of leaves of five cucumber cultivars and T. urticae moving stages infestation during 2009 season.

Table 4 :
Relation between anatomy of leaves of five cucumber cultivars and T. urticae moving stages infestation during 2010 season.