The fungus Corynespora cassiicola causes target spot in ipê trees in Brazil

Researchers confirmed Corynespora cassiicola as a causal agent of target spot in seedlings of Handroanthus serratifolius and Tabebuia rosea in Brazil. The record involves plants in a forest nursery in São Lourenço da Mata, Pernambuco. The work represents the first worldwide report confirmed by molecular phylogeny for these two hosts.

Leaves showing symptoms were collected in March 2023. The seedlings presented dark brown necrotic spots with grayish centers. The symptoms had a typical target spot pattern. The nursery maintained approximately 500 seedlings of each species. The incidence of the disease reached 80 percent.

Handroanthus serratifolius, known as yellow ipê, and Tabebuia rosea, pink ipê, have ecological and economic importance in Brazil. The species occurs in urban landscaping and native vegetation. Despite its hardiness, it can suffer from diseases in urban areas, natural ecosystems, and nurseries.

Three leaflets

Scientists collected three leaflets from each host species. Symptomatic fragments underwent surface asepsis in 70 percent ethanol for 1 minute, 1 percent sodium hypochlorite for 1 minute, washing in sterile distilled water, and drying in a laminar flow hood. Then, the fragments were transferred to potato-dextrose-agar medium with 0,5 grams per liter of streptomycin sulfate.

The plates were kept at 28 degrees Celsius, with a 12-hour photoperiod. After 21 days, the researchers obtained six monosporic isolates. The colonies presented a dark brown color, velvety texture, well-defined margins, and abundant aerial mycelium.

Morphological characterization

Morphological characterization indicated septate, light brown hyphae. Conidiophores appeared macronematic, mononematic, solitary, straight to slightly curved, smooth, and tapered. Conidiogenous cells presented a monophialidic pattern. Conidia appeared solitary or in chains, straight to slightly curved, obclavate, light brown, and with four to ten pseudosepta. Measurements ranged from 63,5 to 234,7 micrometers by 7,2 to 9,6 micrometers.

Based on these characteristics, the isolates received initial identification as Corynesporaspp. The observed characters coincided with descriptions of Corynespora cassiicola, especially due to the morphology of the conidia, septation, pigmentation and structure of the conidiophores.

Two isolates were sent for molecular analysis. CFC 1784 came from Handroanthus serratifolius. The CFC 1785 came from Tabebuia rosea. Researchers extracted total DNA and amplified ITS, TEF-1α, TUB2, and LSU regions. The sequences were deposited in GenBank.

Searches for BLAST showed high similarity to isolates of Corynespora cassiicola. The ITS sequences showed 100 percent similarity. The TEF-1α sequences also showed 100 percent similarity. The TUB2 sequences reached 99,77 percent similarity. The LSU sequences reached 99,55 percent similarity.

Phylogenetic analysis

The maximum likelihood phylogenetic analysis used combined sequences from the four markers. The tree grouped isolates CFC 1784 and CFC 1785 with Corynespora cassiicola CBS100822. The clade received 93,9 percent support. The isolates were deposited in the Cariri Phytopathogenic Fungi Collection, at the Federal University of Cariri, in Crato, Ceará.

The pathogenicity tests involved healthy plants of Handroanthus serratifolius and Tabebuia rosea, one-year-old plants grown in plastic pots were sprayed with 10 milliliters of conidial suspension at a concentration of 2 × 10⁴ conidia per milliliter. Control plants received sterilized water.

Each isolate and the control treatment consisted of ten plants per species. The plants were covered with plastic bags to simulate a humid chamber, with approximately 85 percent relative humidity, for seven days. Afterward, they remained in a greenhouse at a temperature of 24 to 30 degrees Celsius.

Two months after inoculation, the inoculated leaves exhibited symptoms similar to those observed in the nursery. The control plants remained asymptomatic. The pathogens were re-isolated and maintained the same morphology as the inoculated isolates. Thus, the tests fulfilled Koch's postulates.

The researchers report a previous account ofCorynespora cassiicolaassociated with leaf spot in Handroanthus serratifolius and Tabebuia rosea in Brazil, a study published in 1980 identified that record based solely on morphological characteristics. The new study adds confirmation through multilocus molecular phylogeny.

The study was conducted by scientists Ingryd Luzia de Farias Andrade, André Nunes de Oliveira, Ana Gabriele Gurgel Amaral, Kamila Câmara Correia, Rosana Blawid, Humberson Rocha Silva, Marcos Paz Saraiva Câmara, and Sami Jorge Michereff.

Further information can be found at doi.org/10.1111/jph.70329

Why does this matter?

The discovery that the fungus Corynespora cassiicola - the causative agent of target spot in soybeans - infects yellow and pink ipê trees and is important for soybean farmers. Target spot causes losses of up to 40% in soybean productivity in Brazil (DOI: 10.1111/ppa.12925).

Native species, common in nurseries and rural areas, can act as perennial reservoirs of the pathogen, contributing to the maintenance of inoculum between harvests.

This is the first molecular confirmation of the disease in these hosts worldwide. The finding expands knowledge about the broad spectrum of hosts of Corynespora cassiicola and its epidemiology, supporting more comprehensive integrated management strategies, such as monitoring ornamental plants near crops and crop rotation.

The discovery does not change immediate control practices, but it strengthens the scientific basis for protecting soybeans more effectively.