Guava Decline: A Complex Disease Involving Meloidogyne mayaguensis and Fusarium solani

In Brazil, Meloidogyne mayaguensis has become a threat to guava production. Approximately a third of the cultivated area is infested, leading almost inevitably to the decimation of the orchards. Because parasitized trees develop rotten roots as the disease progresses, the possibility that a soil‐borne pathogen could be involved was investigated. From several nematode‐free or nematode‐infested orchards, nearly 2000 root fragments were tested for bacteria and fungi. Positive isolations were obtained from nematode‐infested areas only and were predominantly identified as Fusarium sp. In a 5‐month microplot experiment, guava seedlings were uninoculated (control) or were inoculated with M. mayaguensis only or with this nematode and 21 days later with one of 11 Fusarium sp. isolates. A Scott–Knot analysis of several vegetative variables and of the extent of root rot allowed the generation of a dissimilarity dendrogram that indicated that four Fusarium sp. isolates were particularly associated with damage to the seedlings. Upon identification of these isolates as Fusarium solani, a 6‐month microplot experiment was set up, in which guava seedlings were uninoculated or were inoculated with one of the following: (i) M. mayaguensis only, (ii) four F. solani isolates, separately, (iii) four F. solani isolates separately, combined with physical injury of the roots with a knife, (iv) M. mayaguensis, and 21 days later with four F. solani isolates, separately. No root rot and virtually no effect on all variables were observed in the seedlings inoculated with the fungus isolates, with or without physical injury. Major root rot and a negative effect on all variables were observed in the seedlings inoculated with M. mayaguensis and all four F. solani isolates. This characterizes guava decline as a complex disease caused by the synergistic effect of these organisms, in which parasitism by the nematode predisposes the plants to root decay caused by the fungus.