Original Article

http://opn.to/a/dRzaU

Speckle by Corynespora torulosa (Syd.) Crous: a pre-harvest fruit disease of Musa spp. in Cuba

Punteado por Corynespora torulosa (Syd.) Crous: una enfermedad de precosecha de los frutos de Musa spp. en Cuba


ABSTRACT

It was studied the etiology of a fruit disease spots in form of a speckling of Cavendish banana fruits that causes fruit quality deterioration and commercial rejection. For this purpose, peel spots were placed in humid chambers to incite fungal development, and isolations were carried out on water agar + streptomycin. To fulfill Koch postulates, CEMSA 3/4 and Burro CEMSA fruits were inoculated with a spore suspension of 1x105 conidia/ml and incubated at room temperature under high relative humidity. Fruits of Cavendish and Gros Michel (AAA), CEMSA 3/4 and Pisang ceylan (AAB), Pisang awak and Burro CEMSA (ABB) were inoculated as described. The fungus Corynespora torulosa (Syd) Crous, comb. new (syn. Deightoniella torulosa (Syd.), M.B. Ellis) was uniformly recovered from the humid chambers and isolations. The artificial inoculations of Cavendish fruits with the isolates obtained reproduced the speckling symptoms, and C. torulosa was confirmed as the causal agent of speckling. All varieties inoculated showed to be susceptible to the pathogen. It is the first time that C. torulosa is described in Cuba as the causal agent of fruit speckling previously attributed to other causes. C. torulosa is a colonizer of senescent leaves, fruit flowers, and bracts of Musa plants and becomes airborne during appropriate environmental conditions. It is discussed that sanitation of senescent leaves and fruits organs are essential to reduce incidence of the disease.

Keywords: 

Deightoniella torulosa; fruit wastages; sanitation; resistance of cultivars.

RESUMEN

Se estudió la etiología de una enfermedad de los frutos en forma de punteado en frutos de bananos Cavendish que causa deterioro y rechazo comercial de frutos. Se colocaron manchas de la piel de los frutos en cámara húmeda para incitar el desarrollo de hongos y se realizaron aislamientos en agar agua + estreptomicina. Para cumplimentar los postulados de Koch, se inocularon frutos de CEMSA 3/4 y Burro CEMSA con una suspensión de 1x105 conidios/ml y se incubaron a temperatura ambiente y alta humedad relativa. Se inocularon frutos de Cavendish y Gros Michel (AAA), CEMSA – y Pisang ceylan (AAB), Pisang awak y Burro CEMSA (ABB) de la forma ya descrita. De las cámaras húmedas y aislamientos se recobró uniformemente el hongo Corynespora torulosa (Syd) Crous, comb. new (syn. Deightoniella torulosa (Syd.), M.B. Ellis). Las inoculaciones artificiales de los frutos de CEMSA 3/4 y Burro CEMSA con los aislamientos obtenidos, reprodujeron el punteado de los síntomas y se confirmó que C. torulosa es el agente causal del punteado de los frutos. Todas las variedades inoculadas mostraron ser susceptibles al patógeno. Esta es la primera confirmación en Cuba de que C. torulosa es el agente causal del punteado de los frutos, antes atribuido a otras causas. C. torulosa es un colonizador de hojas senescentes, flores y brácteas de plantas de Musa spp. Se discute que el saneamiento de hojas viejas, y órganos de los racimos es esencial para reducir la incidencia de la enfermedad.

Palabras clave: 

Deightoniella torulosa; daños a frutos; saneamiento; resistencia de los cultivares.


Banana and plantain are important crops in economic, food sustainability and social terms in Latin America and Caribbean countries (LA & C; 1,2,3,4). In Cuba, banana and plantain are also important due to the daily consumption and as cash crops that every week render benefits to the growers. Banana and plantain cultivated surface and production in Cuba in 2017 were (5): banana 24,046 ha and 295,526 t. and plantain 64,103 ha and 719,039 t. Diseases are important constrains to banana and plantain production and pre- and postharvest diseases reduce commercial yield, revenue, and profitability. Banana and plantain pre- and postharvest fruit diseases have been exhaustively studied in Cuba (6,7,8).

In the present paper, descriptions are made of the symptoms and etiology of a pre-harvest fruit spot found on Cavendish fruits in Artemisa province of Cuba in April 2019, as well as of the pathogen morphology and its pathogenicity on banana and plantain. The epidemiology and management practices are discussed.

Fruits of Cavendish banana plants with a speckling symptom were collected and taken to the plant pathology laboratory of INISAV to describe the symptoms and isolate and describe the pathogen.

Symptoms were described and photographed. To determine etiology, fruit peel with spots were washed under tap water, disinfected with alcohol 70 % for 1 min, washed three times with sterile distilled water and placed inside of sterile 100 mm Petri plates on sterile humid filter paper. Afterward, the plates were incubated at room temperature in moist chamber (100n % relative humidity) and observed daily until a fungal structure developed.

Pathogen isolation was carried out from the disinfected peel spots by placing 2 - 3 mm peel pieces from the border of the spots on water agar (16 g agar Biocen/L distilled water) + streptomycin 50 µg/ml. The plates were incubated at 29ºC and observed daily for fungal growth. Once noticed the fungal growth, it was transferred to Petri plates with PDA and incubated in the dark at 29ºC. The morphology of single spore colonies, the pigmentation, development of conidiophores and conidia, were described, and 50 conidia and conidiophores were measured at 400x magnification under a Carl Zeiss microscope and compared with CMI pathogenic fungal sheets descriptions. In order to fulfill Koch postulates, five individual fruit fingers of CEMSA 3/4 (subgroup Plantain, AAB) and Burro CEMSA 3/4 (subgroup Bluggoe, ABB) cultivars of approximately 40-50 days old were inoculated with a conidia suspension of 1x105 conidia/ml in sterile water, prepared from a ten-day-old single spore colony of the fungus grown on PDA (20 g Oxoid agar / L of water) and incubated in humid chambers prepared in trays with humid filter papers at room temperature as above described to induce symptom development and sporulation.

Pathogenicity tests to different banana and plantain genotypes were carried out with Grand nain (subgroup Cavendish, AAA), Gros Michel (subgroup Gros Michel, AAA), CEMSA 3/4 (subgroup Plantain, AAB), Pisang ceylan (subgroup Pisang ceylan, AAB) Manzano vietnamita (subgroup Pisang awak, ABB) and Burro CEMSA (subgroup Bluggoe, ABB). Conidial suspensions were prepared from seven-dayold single spore cultures grown on PDA and incubated as described for ten days. Fruit fingers of each cultivar were surface disinfected with 70 % ethylic alcohol, and three squares of 1 cm2 were marked on each fruit with a felt pen and delimited with fused paraffin. Two of the squares were inoculated by depositing a drop (0.2 µl) of a spore suspension (1x105 conidia/ml) inside them, and one sterile water drop were deposited in the third square as control. Three replications with two fruits of each cultivar were used in a completely randomized design assay. Fruits were incubated in trays covered with humid filter papers and observed daily until symptom development.

The symptoms found in the field consisted of numerous superficial tiny spots on the peel that did not affect pericarp of the fruit. Individually, the spots were round and firstly developed as a watery halo on the peel that became reddish brown and later necrotic, more frequently with a 2-3 mm diameter that could reach 4 mm and surrounded by an external halo (Figure 1). Symptoms were more abundant close to the neck or peduncle of the fruit and on the inner side than on the external side of the fruit.

Figure 1. 

Symptoms of speckling on Cavendish banana fruits; A) first symptoms as a halo; b) typical spots as specks surrounded by a halo: C) presence of mature spots of different ages and size. / Síntomas de punteadura en frutos de bananos Cavendish: A) primeros síntomas en forma de halo; B) manchas típicas de pecas rodeadas de un halo; C) presencia de manchas maduras de diferentes edades y tamaños.

On the spots incubated in humid chambers, conidia developed at the extreme of long conspicuous brown conidiophores after 48 hours of incubation. From the spot tissues on agar, a single fungus developed that, after being isolated and incubated on PDA, produced initially clear gray colonies that turned dark gray and black with the presence of abundant characteristic brown conidiophores identical to those developed in humid chambers. At the extreme of hyphae, 14.64 - 43.92 x 2.44 - 4.88 µm pale brown conidiophores, swollen at the apex, developed single or in small groups with successive proliferations (Figure 2). Conidia developed singly at the ends of the conidiophores and their successive proliferations. Conidia were 45-75 (X= 57,7) x 10-20 (X=16.6) µm straight or slightly curved, obpyriform to obclavate, hyaline to gray olive more intense pigmented close to the scar, 1-5 pseudoseptate (more frequently 3 pseudosepta).

Artificially inoculated fruits develop typical speckling symptoms in 4 days (see Figure 3)