Effects of chemicals and cultivar on nematodes and fungal pathogens of citrus roots

G. E. Walker

Abstract


Citrus rootstocks sweet orange (SO) and carrizo citrange (CC) were grown in pots in two naturally infested Renmark citrus soils (A, infected with Tylenchulus semipenetrans, Paratrichodorus lobatus, Xiphinema americanum s.l., and Pythium ultimum; B, infected with all of these pathogens, except for Xiphinema sp., and with Phytophthora citrophthora). After planting, soil A was either drenched with water (as a control), solutions of furfural, or acetone-extracts of eugenol or camphor, or granules containing cadusafos were spread on the soil surface and then irrigated with water. After planting, soil B was drenched with water, with solutions of oryzalin, or with thiophanate/etridiazole +/– cadusafos granules, or a foliar spray was applied of phosphonate +/– cadusafos granules. The drenches containing thiophanate/etridiazole and oryzalin were re-applied after 12 weeks. After 24 weeks of growth, shoot and root growth were greater in CC compared with SO in both soils. Densities of T. semipenetrans females were lower on CC compared with SO roots in soil A, but not soil B, indicating that these populations differed in virulence. Soil densities of the other nematodes did not differ between cultivars. Isolation frequency of Phytophthora citrophthora, but not Pythium ultimum, was lower on CC roots, and root rotting was more severe in SO. Eugenol stimulated shoot growth, but furfural inhibited both shoot and root growth. In soil A, only cadusafos reduced root density of T. semipenetrans and soil density of P. lobatus, but all treatments reduced soil density of Xiphinema sp. Oryzalin, and thiophanate/etridiazole + cadusafos, stimulated root growth of both cultivars, and the latter treatment stimulated citrange shoot growth. Oryzalin, and either thiophanate/etridiazole or phosphonate + cadusafos, reduced the severity of root rot. However, phosphonate inhibited root growth of both cultivars and citrange shoot growth, indicating a sensitivity in young seedlings. In soil B, only cadusafos treatments reduced root density of T. semipenetrans. These treatments also reduced soil density of P. lobatus but oryzalin increased soil density of P. lobatus. Isolation frequency of Pythium ultimum from roots was higher from trees treated with thiophanate/etridiazole +/– cadusafos, and isolation frequency of Phytophthora citrophthora was higher from roots of trees treated with thiophanate/etridiazole + cadusafos. Eugenol shows promise as a growth stimulant for citrus but eugenol, camphor and furfural were much less effective as nematicides than cadusafos.

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