# Evaluation of Fire Blight Resistance of Eleven Apple Rootstocks Grown in Kazakhstan Fields
## 1 Introduction  
Apple is one of the most economically important pome fruit crops in the world. In Kazakhstan, the fields occupied by apple gardens amount to 35.7 thousand hectares, which is 75% of the total field area used for the propagation of stone and pome fruit crops in the country, according to data from the Bureau of National Statistics Agency for strategic planning and reforms of the Republic of Kazakhstan (2023).  
The pandemic bacterial disease infecting pome fruit is fire blight, which can cause devastating yield losses [1; 2]. The causal agent of fire blight is the enterobacterium _Erwinia amylcorora_ ([PERSON]) [PERSON] et al. [3]. This pathogen can infect flowers, fruits, shoots, woody tissues, and rootstock crowns of the apple tree, resulting in tree death in cases of severe infection [4].  
The control of fire blight epidemics is crucial not only for commercial fruit production but also for biodiversity safety. The distribution of fire blight in wild populations of apples is an especially important concern [5].  
The propagation of resistant host plants is considered the most effective strategy for managing fire blight, especially since quantitative trait loci (QTLs) linked to fire blight resistance have been identified in several wild and cultivated _Malus_ species, with phenotypic variance ranging from 15 to 83% [6; 7; 8; 9; 10; 11]. However, the introduction of resistance genes from wild species into apple varieties is a time-consuming process; indeed, it can take decades due to the long generation times of apple trees and multiple pseudo-backcrosses [12]needed to eradicate the undesirable fruit qualities of wild apples, such as small size and bitter or asringent flavors. Because of this, most commercially successful apple cultivars are still susceptible to fire blight [13].  
Resistant wild accessions are likewise used in rootstock breeding programs worldwide. The fire blight-resistant rootstocks of the Geneva series G.11, G.41, G.202, G.214, G.890, G.935, and G.969 [14] are descendants of the wild apple _Malus \(\times\) robusta 5_ (Mr5) that bears a resistant QTL on LG3 [15; 16]. Nevertheless, the moderately resistant M.7, the Budagovskij series B.9 and B.118, and the very susceptible Malling series M.26 and M.9 [16] are widely used in the grafting of commercial cultivars due to their high-quality horticultural traits.  
Molecular methods enable the rapid and reliable identification of resistant genotypes for marker-assisted selection [17]. Identification of loci associated with fire blight resistance in apples is made possible by SSR [15], SCAR [18], and SNP [19] markers. The use of whole-genome sequencing (WGS) offers a great opportunity to not only confirm the presence of all resistant alleles but also to discover new ones [20; 21]. Nevertheless, WGS is expensive and, therefore, not yet widely used in the screening of plant genotypes for breeding.  
The recently developed SNP panel for the identification of disease resistance and horticultural traits in apples was validated by screening 240 phenotyped individuals of _Malus \(\times\) domestica_ Borkh to use in marker-assisted selection [19]. This panel includes five SNP markers of loci on LG3 that explain up to 80% of phenotypic variation in fire blight resistance [15]. These five SNP and SCAR markers for evaluating the presence of loci FB_F7 on LG7 [10] were used in the present study for testing eleven rootstocks commonly used for commercial apple grafting in Kazakhstan. An investigation of the connection between genetic background and environmental conditions was the main purpose of our two-year monitoring survey, which enabled us to distinguish whether rootstocks growing in the climate conditions of the Kazakhstan southern region are either susceptible or resistant to fire blight.