Control of fungal diseases by compounds acting as antipenetrants

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Antipenetrant action is the basis of disease control by non-fungitoxic concentrations of melanin biosynthesis inhibitors such as tricyclazole, pyroquilon and fhthalide. These compounds block the polyketide pathway, thus preventing melanization of the appressorial walls of Pyricularia oryzae and Colletotrichum species. Non-melanized appressorial walls of these pathogens do not appear to have the proper architecture and rigidity needed to penetrate plant epidermal walls. Moreover, polyketide metabolites which accumulate as a result of inhibition of melanin biosynthesis may interfere with the appressorial penetration processes in P. oryzae. Melanin biosynthesis inhibitors are important protectants used for the control of rice blast disease caused by P. oryzae. Cutinase inhibitors are another group of compounds which prevent infection at non-fungitoxic concentrations, apparently by specifically inhibiting cutinase activity. Antipenetrants in this group include diisopropylfluorophosphate and paraoxon. The organophosphate fungicides IBP (Kitazin) and edifenphos (Hinosan) are cutinase inhibitors and display a specific antipenetrant action, but in practice their disease control activity may also involve direct fungitoxicity. The fungicide benomyl degrades to the fungitoxic product carbendazim and butylisocyanate. Superior performance of benomyl as compared to that of carbendazim, which is sometimes observed in disease control, possibly results from the inhibition of cutinase by butylisocyanate. The antibiotic, validamycin A, does not inhibit production of mycelial mass by Rhizoctonia species but blocks hyphal extension and increases branching. This antibiotic may act as an antipenetrant toward Rhizoctonia-type fungi. The development of antipenetrant chemicals offers an interesting approach as a mechanism for control of plant fungal diseases.