Suppression of Phytophthora Infection in Citrus Infected with Viroids

Citrus viroid-induced resistance to Phytophthora infection in citrus was measured by the number of Phytophthora sporangia in ‘Rio Red’ grapefruit (Citrus paradisi Macf.) bait tissue infected with citrus viroids compared with non-inoculated controls. Different viroid isolates containing mixtures of viroids [Citrus exocortis viroid (CEVd), Hop stunt viroid (HSVd), Citrus viroid III (CVd-III), Citrus viroid IV (CVd-IV)] were designated by plant numbers and sources. Source 13E was associated with the lowest number of sporangia in bark, leaves, and roots used as baits, whereas CEVd E9, a known severe CEVd isolate, significantly reduced the number of sporangia in leaves and bark. Sources 1A, 2E, 3E, 4D, and 6E showed a significantly reduced number of sporangia on bark, leaves, and roots compared with healthy plants and 44A; however, their effect was not as pronounced as that of E9 and 13E. Sources 12E and 44A did not suppress sporangia production. Previous reverse transcriptase–polymerase chain reaction analysis showed that all source plants had mixed infections with several viroids, whereas 12E and 44A contained no viroids. In addition to confirming the earlier reports on the suppression of Phytophthora infection in general, our study showed significantly reduced Phytophthora sporangia development resulting from a number of viroids in mixed infection, but there did not appear to be any effect related to viroid species. To determine if concentration affected resistance to Phytophthora, phenolic acids were extracted. Extraction of phenolic acids with 80% ethanol was more efficient compared with 100% methanol and an acetonitrile–water mixture. High-performance liquid chromatography revealed no notable detection of salicylic acid in healthy and viroid-infected plants, but there was a small peak corresponding to salicylic acid in Phytophthora-infected and both viroid and Phytophthora-infected plants. Flavone was detected in all the source plants with a slight increase in Phytophthora-infected and both viroid and Phytophthora-infected plants. A peak corresponding to quercetin dehydrate was detected in Phytophthora-infected plants. Efficient use of the right viroid isolate(s) can result in suppression of Phytophthora infection of citrus.