# Evaluation of Fire Blight Resistance of Eleven Apple Rootstocks Grown in Kazakhstan Fields
## 4 Discussion  
Clonal rootstocks of apple trees are one of the main components of intensive horticulture, and they play an important role in regulating tree growth, yield, fruit quality, and resistance to biotic and abiotic stresses [40; 41]. Since 1917, when apple rootstock breeding began at the East Malling Station, the development of improved rootstock types in many traits has continued. In the process of apple rootstock breeding, it is crucial to initially focus on horticultural characteristics, such as grafting compatibility, reproductive ability, and dwarfism, and their effects on scion growth. Regarding the area for planting apple trees, significant attention should be focused on both environmental factors and biological factors such as viruses, bacteria, nematodes, and pests [42]. European countries have mostly focused on the breeding and planting of dwarf rootstocks, which have advantageous characteristics in terms of production, fruit quality, resistance to diseases and pests, and competition with weeds for water and nutrients [43]. In Eastern Europe, farmers have additionally sought to breed rootstocks with cold tolerance and easy propagation [44]. In North America and Canada, the focus has been mainly on dwarfism and freezing tolerance [29]. In Kazakhstan, farmers have valued rootstock  
Figure 4: Resistance of apple rootstocks with respect to genetic background. The green color indicates resistant rootstocks; light orange indicates susceptible MM106. types that have advantageous characteristics in fruit quality and cold tolerance. Only recently, they have sought to propagate disease-resistant rootstocks. The main diseases spread in apple gardens in Kazakhstan are fire blight [5; 43; 45], apple scalp [46; 47], powdery mildew [48; 49], and diseases of viral origin [50; 51; 52]. Because fire blight is widespread in Kazakhstan, farmers have aimed to cultivate apple trees comprising rootstock and scion that both have pathogen resistance. LG3 [15], LG12 [7; 53], and LG10 [8], derived from wild apples, are loci that have been allocated and confirmed as conferring fire blight resistance. Moreover, the QTL FB_F7, identified on LG7 of the domesticated apple cultivar 'Cox's Orange Pippin', has been described as the most promising for breeding because of its moderate and stable response to disease [10; 18]. Since the QTLs on LG3 and FB7 explain a significant level of phenotypic variation, ranging from 67% to 83% and 35% to 40%, respectively, they were selected for testing using already validated markers [9]. Moreover, the local rootstocks were derived from genotypes of wild apples, such as _M. sieversii_ and _M. niedzwetzkyana_, in which FB7 has been previously identified [54; 55]. Clearly, the local and Russian rootstocks were not extensively tested for other markers associated with resistance to fire blight. In the present work, 'MM106', which was positive for marker AE10-375, was infected in both 2021 and 2022, whereas G.41, which was negative for QTL FB_F7 and positive for five SNP markers associated with LG3, was resistant. The presence of resistance alleles for both markers AE10-375 and GE-8019 in the genotypes has previously been shown to indicate high resistance to the fire blight pathogen [10], but the rootstock types analyzed here were found to be negative for marker GE-8019. A similar level of spreading and severity of disease was observed for both positive and negative AE10-375 rootstock types; however, rootstocks additionally bearing resistant alleles for SNP markers, such as G.16 and '62-396', showed no sign of infection in the fields. The resistance of G.41, G.16, and B.9 to fire blight was previously demonstrated in field surveys by grafting different cultivar scions [30; 56; 57; 58]. In the current study, a restrained distribution of disease was shown only for the G.41, G.16, and '62-396' fields, while the B.9 field was the most infected.