First report of angular leaf spot in Acmella oleracea caused by the foliar nematode Aphelenchoides pseudobesseyi

Abstract

Jambu plants (Acmella oleracea) exhibiting necrotic angular leaf spots were collected in Belém—Pará, Brazil. Suspected to be a disease caused by nematodes, the jambu leaves were processed and the presence of nematodes from Aphelenchoides genus was observed using a light microscope. These nematodes were reared on Fusarium sp. and subsequently morphologically and molecularly characterized to species level. Aphelenchoides sp. associated with jambu exhibited morphological and morphometric characteristics similar to those within the A. besseyi complex (A. besseyi sensu stricto, A. oryzae and A. pseudobesseyi). Still, these characteristics were insufficient to confirm the identity of this population. However, the Bayesian inference analysis, utilizing the expansion segment of the large subunit (D2-D3 LSU) of nuclear ribosomal DNA, yielded results with a high posterior probability, indicating that the Aphelenchoides sp. associated with jambu belongs to the A. pseudobesseyi species. Under controlled conditions, the reproduction of the nematode in the leaf tissues (RF > 1) was observed, resulting in disease symptoms. The highest reproductive rate of A. pseudobesseyi (RF = 2.6) was observed from inoculation with 100 nematodes per leaf (500 nematodes per plant). Based on Seinhorst analysis, the tolerance limit in jambu plants infected with A. pseudobesseyi was 250 nematodes. It is concluded that A. pseudobesseyi is the etiological agent of the jambu angular leaf spot. For future evaluation of jambu resistance on A. pseudobesseyi, an inoculate maximum of 100 nematodes per leaf is recommended.