บทคัดย่อ
Anthracnose crown rot and fruit rot caused by Colletotrichum gloeosporioides and C. acutatum are two of the most prominent and destructive diseases of cultivated strawberry (Fragaria x ananassa Duchesne). Both species are capable of establishing hemibiotrophic infections (HBI) in leaf tissue, encouraging undetected dissemination of diseased nursery plants to fruit production fields and inciting subsequent crown- and fruit-rotting epidemics. The purpose of this research was to explore a novel mechanism in strawberry foliage, which may confer rate-limiting resistance to multiple Colletotrichum species in that tissue. Germplasm from the NC State strawberry breeding program were screened for resistance to C. gloeosporioides HBI. Preliminary variation in resistance and significant differences among genotypes were observed. Methods were developed to increase the accuracy and precision of percent sporulating leaf area (PSLA) measurements, including the use of imaging software for quantification of PSLA. Direct visual estimates were strongly correlated (r=0.91) to image-based quantification of PSLA, but the accuracy of visual estimation varied with different patterns of sporulation and tended to underestimate higher infection severities. A larger panel of 18 cultivars and NC State selections were screened for resistance to hemibiotrophic foliar infections of both C. gloeosporioides and C. acutatum. Genotype PSLA means were significantly different and ranged from 8.5% to 26.5%, though means separation was poor for moderate genotypes. These 18 genotypes were also evaluated for resistance to anthracnose crown rot (ACR) by applying C. gloeosporioides inoculum directly to the crown and observing wilt symptoms over ten weeks. ACR means separated distinctly over a wide range of resistance phenotypes. Findings of moderate correlation of resistance to C. gloeosporioides and C. acutatum HBI in leaf tissue suggested resistance to these Colletotrichum species may be shared in this common tissue type. However, weak correlation of resistance to C. gloeosporioides HBI and ACR in leaf and crown tissue, respectively, suggested that resistance to a single Colletotrichum species may operate independently between tissue types. A population was constructed to examine the inheritance of rate-limiting resistance to C. gloeosporioides and C. acutatum HBI in strawberry leaf tissue as well as the inheritance of resistance to ACR in crown tissue. Low dominance to additive variance ratios for C. acutatum HBI and C. gloeosporioides ACR indicate strong additive genetic control of resistance to these traits. C. gloeosporioides HBI had a much higher dominance to additive variance ratio which suggested this trait was under nearly equal dominance and additive genetic control. Heritability estimates were low for C. acutatum HBI (0.25) and C. gloeosporioides HBI (0.16) but were moderate for C. gloeosporioides ACR (0.61). Resistance gains from selection were predicted to be high for ACR, moderate for C. acutatum HBI, and low for C. gloeosporioides HBI. A strong genetic correlation (rA = 0.98) between resistance to C. acutatum HBI and C. gloeosporioides HBI suggested that resistance to these two Colletotrichum species was controlled by the expression of common genes in strawberry leaf tissue. Selecting for increased resistance to HBI for one Colletotrichum species should produce gains in resistance to the other. Negative genetic correlations between ACR and both HBI traits (rA = -0.85 and -0.61) suggested that resistance in crown tissue is inherited independently of resistance in leaf tissue in the examined population. These results recommend independent evaluation and advancement of resistance to HBI and ACR within breeding