Selection of arbuscular mycorrhizal fungi for citrus growth promotion and Phytophthora suppression

In order to reduce unnecessary amount of P-fertilizer and severity of Phytophthora root rot in citrus orchards, the experiment was set up. Thirteen indigenous arbuscular mycorrhiza (AM) fungi species were isolated from rhizosphere soil of citrus orchards in Thailand and were then propagated into three host plants [sorghum (Sorghum bicolor), maize (Zea mays), and leek (Allium cepa)] by trap culture. We also tested whether indigenous AMF species (13 different species) could colonize into three cultivars of citrus scions and rootstocks (Shogun: Citrus reticulata Blanco cv. Shogun; Tangerine: C. reticulata; and C-35 citrange: Citrus sinensis × Poncirus trifoliata). With root colonization rates, the results indicated that Acaulospora tuberculata and Glomus etunicatum provided the best colonization in all citrus cultivars. We selected, therefore, those AMF species to verify their influences on citrus growth and Phytophthora root rot resistance. Three cultivars of citrus scions and rootstocks, Shogun, Tangerine and C-35 citrange, were inoculated with two effective indigenous AMF species, G. etunicatum or A. tuberculata in order to determine the influences on citrus growth. The plants were investigated to determine the mycorrhizal efficiency index (MEI), AM colonization, P content, and other parameters. Co-inoculation of AMF species (G. etunicatum or A. tuberculata) with Phytophthora nicotianae was also carried out in Shogun scion/C-35 citrange rootstocks. The results of citrus growth revealed that Shogun and Tangerine inoculated with G. etunicatum produced the highest MEI. Tangerine and C-35 citrange amended with fertilizers and G. etunicatum showed the highest P content in leaves. This indicated that G. etunicatum has an influence on citrus growth and P uptake, suggesting it to be the highly effective strain. Shogun scion/C-35 citrange rootstock combinations that were inoculated by both P. nicotianae and different AM fungi (G. etunicatum or A. tuberculata) showed root injury at low level of root rot symptom. However, the part of Shogun scions grafted on rootstocks showed severe symptom of shoot die back in treatment inoculated with P. nicotianae alone, while treatment inoculated with different AM species (G. etunicatum or A. tuberculata) and P. nicotianae rendered lower shoot die back symptoms than that of Phytophthora treatment. The low level of shoot die back symptom was shown at first, then healthy young shoot was restored. Our results indicated the facts that different host plants and different AMF species produced different outcomes of growth and pathogen resistance. The application of both AM isolates, therefore, has an enormous potential to be produced the inoculum for citrus orchards.

► A. tuberculata and G. etunicatum host plants show the highest AM colonization.
► Isolated AMFs associate into citrus roots with high AM colonization.
► Selected AMFs provide positive effect on citrus growth.
► Selected AMFs reduce severity of Phytophthora root rot.