Fusarium solani species complex newly identified to cause root rot in hydroponically grown lettuce and cilantro in Puerto Rico

Commercial production of cilantro (Coriandrum sativum) and lettuce (Lactuca sativa) in Puerto Rico mainly takes place under hydroponic systems in 30-35 day growth cycles. Approximately 120 producers grow 200 ha (500 acres) of cilantro and lettuce year around representing $3.7M and $216,000 dollars, respectively in 2006. Under these conditions both crops are affected by diseases such as damping-off caused by Pythium and leaf spots caused by Alternaria, and lettuce only by leaf spot caused Cercospora.

In 2006, two hydroponic systems in Lares and Adjuntas experienced 100% losses in cilantro and lettuce due to root rot. Symptoms included leaf chlorosis, stunting and necrosis in the vascular tissue. In both crops, isolations from infected tissue on potato dextrose agar (PDA) resulted in white to cream aerial mycelia with sporodochia of blue-green color. Abundant chlamydospores were single or in pairs on terminal or lateral hyphae. On carnation leaf agar microconidia were formed on long monophialides, 2.2-5 x 8.7-15.6 µm in size. Macroconidia born on short conidiophores were curved, thick walled, 3-5 septate, 3.4-5.8 x 28-52 µm long. The isolates were identified as Fusarium solani species complex based on morphological and cultural characteristics (Nelson et al., 1983). This was confirmed by amplification of the internal transcribed spacer ribosomal DNA (ITS rDNA) that resulted in a 100% match with Fusarium sp. (GenBank Accession No. EF687945), and 99% similarity with specimens in Fusarium solani species complex. A sequence of isolate 6.M8 obtained from cilantro has been deposited in the GenBank under Accession No. FJ224382.

Pathogenicity tests were conducted by dipping the roots of seven day old plants in a suspension of macroconidia with 10⁵ spores/ml (cilantro 'Leisure' and lettuce 'Del Pais'). Similar symptoms to the ones observed in the hydroponic systems developed five to seven days after inoculation. Fusarium solani species complex was re-isolated from the roots. All controls remained healthy. Fusarium sp. has been reported previously in cilantro in Puerto Rico causing root rot (Almodóvar, 1998, 2008). Fusarium spp. are a recurrent problem in hydroponics in Puerto Rico, accounting for 67 samples of cilantro submitted to the Plant Disease Clinic over the last four years. However, the species has not been identified until now. Because the incidence of root rot in hydroponics is 100% and the use of fungicides is restricted, alternative control measures are urgently needed. This is a first report of Fusarium solani species complex in cilantro and lettuce under hydroponic systems in Puerto Rico.