RESISTANCE TO ANGULAR LEAFSPOT DISEASE OF STRAWBERRY: INFLUENCE OF SEEDLING AGE

Abstract

The bacterium Xanthomonas fragariae (Xf) causes the economically important angular leafspot disease to which a high level of resistance has not been found within the cultivated strawberry Fragaria ×ananassa. The 2002 introduction of resistance donors ‘US 4808’ and ‘US 4809’ by the Fruit Laboratory at the Beltsville Agricultural Research Centre, provided sources of resistance at the octoploid level derived from F. virginiana. Moving this resistance through three generations of back crossing to different recurrent F. ×ananassa parents along with screening progeny (12-week-old seedlings) for resistance with a spray inoculation method, followed by selection based on horticultural traits, has created improved resistant genotypes. Selection from mature plants of seedling populations that had not been screened for Xf resistance gave 50% resistant selections – much higher than expected. Therefore, an experiment was designed to determine if some plants develop resistance after 12 weeks of age. Seven controlled crosses were performed in the greenhouse including three resistant (R) × resistant, three resistant × susceptible (S), and one S×S. The S×S cross was included to provide susceptible seedlings, to ensure that the inoculation procedure was effective. Four replicates of seedling sets of 10 to 12 plants per cross were inoculated at 12 or 17 weeks and rated for disease 3 weeks later. The R×R seedlings inoculated at 12 weeks had a mean disease rating of 1.19 compared to 2.28 for seedlings inoculated at 17 weeks. The R×S seedlings inoculated at 12 weeks had a mean disease rating of 2.75 compared to 3.56 for seedlings inoculated at 17 weeks. When seedlings were classified as resistant or susceptible, data analysis showed no significant differences in percent resistant between the inoculation times. These results argue against our hypothesis that older plants show resistance not identified in young seedlings and validate our procedure of screening young seedlings grown to the four to five trifoliate leaf stage. Resistant to susceptible segregation ratios of approximately 3:1 and 1:1 were found in R×R and R×S progeny, respectively, indicating that resistance shows a measure of dominance over susceptibility.