RESPONSE TO APPLE SCAB AND POWDERY MILDEW ATTACK OF DIRECT AND RECIPROCAL F 1 APPLE HYBRIDS

One of the most important methods to create variability in apple breeding, in order to obtain new valuable cultivars, remains intraspecific hybridizations and, the success of creating new apple cultivars depends on the availability of sufficient genetic diversity. The response to apple scab and powdery mildew attack of 469 F1 hybrids (in 2 nd -3 th year of age) belonging to six combinations were analyzed. The response of hybrids to diseases attack was assessed depending of the genitors and type of hybridization (direct and reciprocal crosses). Under natural conditions of infection, without fungicide treatments, a high variability was recorded inside the cross combination. For apple scab, the coefficient of variability (CV%) had values between 12.2% (‘Crimson Crisp’ × ‘Topaz’) and 55.5% (‘Ariane’ × ‘Gala’), and for powdery mildew attack between 68.3% (‘Crimson Crisp’ × ‘Topaz’) and 85.6 % (‘Gala’ × ‘Ariane’). For some apple varieties (i.e. 'Topaz', 'Ariane'), the immunity or the monogenic resistance to scab has not been confirmed, and the heterosis in the hybrid progeny was calculated, being a hypothetical model of quantitative heredity, in order to identifying the progeny with high heterosis effect on scab and powdery mildew resistance. Cuvinte cheie: hibridare, puieți hibrizi, rapăn, făinare


Introduction
Apple scab, caused by Venturia inaequalis (Cke.)Wint., and powdery mildew, caused by Podosphaera leucotricha (Ell.et Everh.)Salm., are the main fungal diseases which are controlled with considerable difficulties and costs, but with long-term consequences and negative effects on the environment and on the fruit quality (Berriea and Xua, 2003;Holb, 2009;Sestraș et al., 2011) Intraspecific hybridization is one of the most used method to create variability in apple breeding.Due to its extensively practice and especially as the result of choosing a confined base for genitors, as there were tendency to use the most known apple cultivars for fruit quality, meaning a relatively small number of options, and thus leading to a narrow genetic basis (Kellerhals et al., 2004), in the process of apple selection the threat of losing genetic diversity is a continuous concern (Sestras, 2004).Obtaining new resistant and valuable genotypes is even more challenging as most of the economical traits of interest in apple are of quantitative nature determined by polymorphism (Dan et al., 2015).In addition, resistant apple cultivars are necessary for organic and integrated fruit growing system, since many of the highly appreciated commercial cultivars are very susceptible to apple scab and powdery mildew attack (Sestraș et al., 2011).This study aimed to analyze the resistance response to apple scab and powdery mildew attack of F 1 hybrids obtained inside artificial crosses (direct and reciprocal) using donors for fruit quality and for diseases resistance.
The assessment of the response to apple scab (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha) has been performed under natural infection environment, in the absence of phytosanitary treatments, every hybrid from the hybrid nursery being rated regarding the attack on leaves.
For scab infection on leaves, the used assessment system has comprised scores (1-9) corresponding to the following levels: 1 = no visible symptom; 2 = a few small scab spots are detectable on close scrutiny of the tree; 3 = scab immediately apparent, with lesions very thinly scattered over the tree; 5 = infection widespread over the tree, majority of leaves with at least one lesion; 7 = heavy infection, multiple lesions or more large surfaces covered by scab on mostly leaves; 9 = maximum infection, leaves black with scab (Lateur and Populer, 1996).Response to primary infection in the spring with powdery mildew on top of twigs, was assessed using 1-9 scale, using following marks: 1 = no visible symptom; 2 = one or few organs affected, detectable on close scrutiny of the tree; 3 = infected organs readily apparent, but without important consequences for the tree; 5 = primary mildew widespread over the branches, inducing the infection of a substantial part of the crown; 7 = heavy infection, half of the organs are badly affected; 9 = crown completely affected, nearly all top of the organs are infected by primary mildew (Lateur, 1999).
The assessment was individually performed for each hybrid per each F1 combination, for a period of two consecutive years (on spring for primary infection of powdery mildew and in the middle of July for apple scab), the hybrids being in their 2 nd -3 th year of age (since their obtaining from seeds).The genitor's response to the diseases attack was assessed in similar conditions with their hybrids, respectively those two-year of observations, under natural infections with no fungicide treatments, the same scale of assessment etc. (the notes for apple scab and powdery mildew attack are presented in Figure 1).For hybrids' combinations, the means of the marks for attack response were compared using one-way ANOVA, followed by Duncan Multiple Range Test.
The level of heterosis (absolute heterosis, relative heterosis and heterobeltiosis) for the marks accorded to diseases attack was calculated.Absolute heterosis values were determined for apple scab and powdery mildew attack in two ways (Baciu et al., 2012): -By comparing the average response of F1 hybrids with parental mean of notes for attack response, using formula: H = F1 -(P1 + P2) / 2.
-By comparing average of F1 hybrids with mean of trait (notes for attack) to the best parent (the parents with lower attack, in this case), using formula: H = F1 -P min .
To calculate relative heterosis (RH), the formula used the average values of F1 hybrids and parents (genitors) H (F P ) / P 100; there was determined also heterobeltiosis (BH) as the percentage of heterosis reported to the best parent according to: BH (F P Min P Min 00; using the average of F1 hybrids and the best genitor (also, with lower value for the reaction to the disease, because the interest is for resistance or tolerance to diseases attack).

Response of F 1 hybrids to apple scab attack
The hybrid combinations with the lowest average of notes for apple scab attack derived from the following crosses: 'Crimson Crisp' x 'Topaz' and 'Topaz' x 'Crimson Crisp' (Figure 2).The higher mean of rates for scab attack were registered in 'Ariane' x 'Gala' and 'Gala' x 'Ariane' combinations.The mean of marks for apple scab attack on leaves was dispersing, with limits ranging from 1.02 ('Crimson Crisp' used as maternal genitor) to 2.25 ('Ariane' used as maternal genitor).The coefficient of variability (CV%) had values between 2.2% ('Crimson Crisp' 'Topaz' and 55.5% ('Ariane' 'Gala' (Figure 3).The results showed that 'Topaz' and 'Crimson Crisp' cvs.transmitted the best response to apple scab attack to their seedlings (both in direct and reciprocal crosses).
One surprising results was obtained especially for 'Ariane' (which was registered with a high note for sensitivity to apple scab), because the cultivar carries the Vf gene.It is know that it is resistant to the five most common races of Venturia inaequalis (races 1 to 5), but tests in the greenhouse showed that it is highly susceptibility to different strains of race 6 (Laurens et al., 2005).Based on two-year field observations of scab symptoms (leaves and fruits) in cv.'Ariane', the mechanism of Vf resistance has been overcome in Romania (Militaru et al., 2017).

Response of F 1 hybrids to powdery mildew attack
The smallest average value of the marks, identifying the hybrids with the best response to the powdery mildew attack, was recorded for 'Gala' x 'Ariane' cross (Figure 4).The highest average value was 4.13 and was recorded for 'Crimson Crisp' x 'Topaz' cross.This sensitive response could be explained by the genitors' influences, 'Crimson Crisp' being known as sensitive to powdery mildew (Biggs et al., 2009).For four cross combinations, the mean of marks was quite close, ranging from 3.16 ('Enterprise' x 'Fuji') to 3.68 ('Topaz' x 'Crimson Crisp').For powdery mildew attack, the coefficient of variability (CV% had values between 68.3% ('Crimson Crisp' 'Topaz' and 85.6 % ('Gala' 'Ariane' (Figure 5).