Pathogen-induced systemic resistance in Ipomoea purpurea.

Infection of Ipomoea purpurea by anthracnose, the disease caused by the fungal pathogen Colletotrichum dematium, increases resistance to subsequent infections on previously uninfected leaves. Fungal isolates varied in their levels of virulence but not in the extent to which they induced resistance. Induced resistance was equally effective against all isolates tested. Plant lines varied in the baseline level of resistance expressed in newly emerging leaves. In some lines, new leaves were poorly defended but developed resistance with maturity, even in the absence of infection. In those lines, induced resistance could not prevent anthracnose damage to young leaves, and this damage reduced plant fitness by increasing juvenile mortality and decreasing juvenile growth rates. In contrast, anthracnose damage to well-defended older leaves had no effect on juvenile growth rates. In at least one line, new leaves were well-defended, regardless of disease experience. This line did not experience reduced growth from anthracnose infection of either young or mature leaves, suggesting that lines with higher baseline levels of resistance are more fit than those dependent upon induced resistance. These results suggest that induced resistance cannot substitute for baseline local resistance in this I. purpurea population.