FUNGAL POLYGALACTURONASE ACTIVITY REFLECTS SUSCEPTIBILITY OF CARNATION CULTIVARS TO FUSARIUM-WILT

Carnation cultivars with different levels of partial resistance were i noculated with race 2 of Fusarium oxysporum f.sp. dianthi and monitore d for accumulation of host phytoalexins, fungal escape from compartmen talization, production of fungal pectin-degrading enzymes and developm ent of external disease symptoms. Accumulation of phytoalexins, assess ed after 10 days in the first 5 cm above the inoculation site, was wea kly (methoxydianthramide S) or not (hydroxydianthalexin B) correlated with resistance levels after 12 weeks. Fungal escape from compartmenta lization, assessed after 3 weeks as percentages colonized plants at 8 cm above the inoculation site, was highly correlated with expression o f susceptibility after 12 weeks. Polygalacturonase (PG) activity, asse ssed after 4 weeks in the first 5 cm above the inoculation site, was h ighly correlated to final disease development, Linear increases in dis ease severity were accompanied by quadratic increases in PG activity. In contrast to water-treated plants, that lacked any PG activity, inoc ulated plants contained two main groups of fungal PGs, the dominant fo rms of which had estimated pi values of 7.0 and minimally 9.5, respect ively. Compared to those of the first group, enzymes of the second gro up were produced only in trace amounts in liquid media containing pect in or polygalacturonate as sole source of carbon. On these media, the fungus also produced a pectin methyl esterase (PME) with an estimated pi of 9.3. Besides PMEs of host origin, inoculated plants of susceptib le cultivars contained the fungal PME while no more than traces were f ound in resistant ones. Assessment of phytoalexin production by the ho st during defense responses cannot replace monitoring of external symp toms as a resistance test. Assessment of fungal growth, whether by rei solations above the compartmentalization area or by measurement of PG activity, provides a both rapid and reliable prediction of disease dev elopment.