RESUMO
Sooty moulds are saprophytic epiphytic fungi that grow mostly on insect secretions, but they can also be associated with plant secretions. In this study, we aimed to describe de interaction of Capnodium alfenasii sooty mould with the extrafloral shoot nectaries of Azadirachta indica. Anatomical and histochemical studies were carried out on serial sections of extrafloral shoot nectaries of A. indica without and with C. alfenasii infestation. The total soluble sugar content of the secreted nectar was determined, and the conidial germination of the fungus in distilled water and in dextrose and nectar solutions was evaluated. The shoot nectaries of A. indica are elongated structures that occur in pairs near the base of the petiole. The exuded nectar contains an average of 534.8 µg of total soluble sugars per µL of nectar and provides ideal conditions for conidial germination and fungal growth. C. alfenasii hyphae grow on the nectary, penetrate through breaks in the cuticle, travel under the cuticle and penetrate the secretory tissue by inter- and intracellular routes. The present report is the first to describe the interaction of C. alfenasii with the A. indica nectary, including the penetration of hyphae into nectariferous tissues and the plant defence mechanisms.
RESUMO
The peach palm (Bactris gasipaes Kunth) is a palm of great importance to the population of the Brazilian Amazon region. Its fruits are an important food source for the local population (Alves and Flores, 1982). Between 2018 and 2021, peach palm fruits with black rot symptoms were collected in the state of Pará, in the municipalities of Juruti (020 09' 08'' S 560 05' 32W) and Santarém (20 26' 22''S 540 41' 55''W), Brazil. Symptomatic fruits detach easily from the bunch. When sectioned, tissues with black coloration and mycelia with white to black coloration were found (Fig. 1a-b). The incidence of the disease in orchards ranged from 5 to 50%. Then, direct isolation, was done by transferring fragments of mycelia and spores to a plate containing potato dextrose agar (PDA). Morphological markers of the asexual phase were evaluated by cultivating the isolates in malt extract agar (MEA) with fragments of Saccharum officinarum culm (Mbenoun et al., 2014). The colonies initially showed a white coloration but turned dark after eight days of cultivation. Colonies produced white mycelia with hyaline, unicellular, rectangular primary conidia (5.6-8.8 µm) (n=30) in length and 2.8-4.0 µm (n=30) in width (Fig. 1e). In the dark-colored mycelia, secondary conidia that formed exhibited three distinct shapes: cylindrical, light brown, and (6.6-11.6 µm x 3.0-3.7 µm) (n=30) (Fig 1. f-i); oblong to globose shape (5.0-10.0 x 3.0-5.3 µm) (n=30) (Fig 1. g); and ellipsoid-shaped (7.0-13.0 x 3.0-4.0 µm) (n=30) (Fig 1. h). Furthermore, unicellular aleuroconidia, produced in dark-colored colonies, exhibited cell walls (10.8-17.5 x 5.4-8.4 µm) (n=30) with a warty, dark-brown, ovoid-shaped appearance (Fig 1. j-k). Genomic DNA was isolated from 4-day-old cultures, and ITS-rDNA and TEF-1α were amplified from ITS1/ ITS2 (White et al., 1990) and EF1F/EF2R (Jacobs et al., 2004), respectively. Sequences were deposited in GenBank (ITS: OL623838, OL623839, OM643316, OL623840, and OL623841) (TEF1: OL631623, OM643318, OM643317, OL631624, and OL631625). Bayesian analysis concatenated were conducted with MrBayes v. 3.2.7 (Ronquist et al., 2011). Clustered the five isolates with the Thielaviopsis ethacetica reference isolate IMI 50560 with Bayesian posterior probability (Bpp = 0.99). (Fig. 2). The pathogenicity test, was conducted using the five isolates, that were were inoculated on six fruits early maturity. After the fruit were washed with neutral detergent and water, 0.5-cm-deep wounds were made with a sterile penknife. Next, 1 mL of primary and secondary conidia suspension at 1 x 105 spores/mL was added to each wound. Six control fruit were inoculated with distilled water. The fruits were kept in incubators at 25 °C with a 12 h photoperiod. The experiment was conducted twice (Alves and Flores, 1982). After five days of inoculation, all inoculated fruit showed characteristic symptoms (Fig 1. c), whereas the control fruit remained asymptomatic (Fig 1. d). The fungus reisolated from all inoculated fruit exhibited the same morphological markers as the inoculated fungus, thus completing Koch's postulates. Thielaviopsis ethacetica is an important pathogen in several palm species in sugarcane and pineapple crops in different parts of the world (Mbenoun et al., 2014; Borgens et al., 2019). This study is the first record of T. ethacetica causing black rot in B. gasipaes fruit in Brazil.