This study reports the occurrence of Age-Related Resistance (ARR) in the potato- P. infestans pathosystem. Six potato varieties with different degrees of resistance to P. infestans were evaluated on the basis of disease severity according to the widely-used Malcolmsons scoring scale at 4, 8 and 12 weeks after plant emergence. The results show that, out of the six varieties, those exhibiting high to moderate levels of resistance to late blight became consistently more resistant as they grew older, while the effect was less pronounced on the two remaining susceptible varieties. Those six varieties exhibited their maximum level of resistance at 12 weeks after plant emergence, when they were just at the bud stage, making the transition to flowering. It is possible that blight resistance in potato plants might be a developmentally regulated response and as the potato plant makes the transition from vegetative to reproductive growth, the re-wiring of certain developmental pathways induces the expression, or results in the accumulation of, certain genes which might also play an important part in defense responses. The data presented in this study emphasizes the importance of considering the age of potato plants when challenged in screening for resistance a well as in determining the most effective timing for use of chemical controls.
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