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Incidence and Biology of Corynespora Cassiicola (berk. & Curt.) Wei. Disease of Okra in Bangladesh

FAKHRUDDIN ALI AHMED
Department of Botany, Jahangirnagar University, Savar, Dhaka

NAZMUL ALAM
Department of Botany, Jahangirnagar University, Savar, Dhaka

ABUL KHAIR
Department of Botany, Jahangirnagar University, Savar, Dhaka

Abstract/ Executive Summary:

Eight commercial okra cultivars were evaluated to determine the magnitude of incidence of Corynespora cassiicola (Berk. & Curt.) Wei. Maximum and significant incidence of C. cassiicola was 14.86 in the cultivar BARI 1. In rest of the cultivars, the occurrence of this fungus was very poor to nil. Corynespora cassiicola appeared to be a minor pathogen for okra. Effects of temperature, pH and culture media on growth and sporulation of the fungus were investigated. Potato dextrose agar medium was found to be the most suitable for optimum growth and sporulation of this fungus at pH 7 and 25ºC. The conidial length significantly varied with the changes of pH. The correlation between pH and conidial length was found negative and highly significant.

Keywords: Incidence, Biology, Corynespora cassiicola, Disease, Okra

Location: Savar, Dhaka

Start Date:

End Date:

Program Area: Pest Management

Commodity:

Objectives:

The present work was undertaken to investigate the disease incidence of C. cassiicola in commercial cultivars of okra as well as to study the biology of the fungus.

Methodology:

Seven okra cultivars, namely BARI 1, Parvanikranti, Chittagong iron, Parash 2, Sabuj bangla, Pan and Sirajuddoula were collected from commercial seed stores of Siddique bazaar, Dhaka, Bangladesh. In addition, an introduced cultivar, ‘red okra’, brought from China was acclimatized in the field condition of Bangladesh and included in the present study. Survey on disease incidence of C. cassiicola was carried out for three consecutive year (2005 to 2007) in the experimental plots at the Botanical garden of Jahangirnagar University following completely randomized block design (RBD). Diseased plants were categorized on the basis of the percentage of diseased surface area (including leaf and stem) according to the scale and formula of Singh (1984). In order to test the pathogenicity, the causal fungus was first isolated from the transitional zone of healthy and diseased tissue on 2% potato dextrose agar (PDA) medium following Dhingra and Sinclair (1985) and was identified on the basis of microscopic characters and cultural behaviour (Ellis 1971, Subramanian 1971). Stems and fruits of okra, grown under restricted condition, were surface sterilized with 0.5% NaOCl then washed with sterilized distilled water. Pathogenicity tests were performed by pricking surfaces of stems and fruits of okra, up to a depth of 0.5 mm, of each of 25 test plants, on which a mycelium plug (2.0 mm in diameter) cut from the edge of a 7-day-old monosporic fungal cultures was placed. In control plants a sterile PDA plug was placed on each wound. To conduct leaf-pathogenicity test, a conidial suspension (ca. 2 × 10-4 conidia/ml) was prepared in sterile water by harvesting conidia from 10-day-old fungal cultures and the suspension was sprayed on to the 25 leaves of three healthy plants. Inoculated plants were kept in humid condition by repeated spraying with sterile water in a polyethylene chamber for the first 48 hrs and thereafter placed in controlled environment. Symptoms similar to those shown by naturally infected plants developed on the wounded stems, fruits and leaves 7 days after inoculation. C. cassiicola was re-isolated successfully from the inoculated plants, confirming Koch's postulates. Control plants remained symptomless. Potato dextrose agar, corn-meal agar, Czapek’s Dox agar, malt extract agar, Richards’ medium, Waksman’s medium and okra leaf extract agar were tested to select the appropriate growth medium for the fungi. Besides, the test fungus was grown on PDA with varying pH and temperature to assess its biological features. PDA medium was adjusted to different pH levels viz., 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 with 1N NaOH or 1N HCl by using pH meter.

Source of Information: Bangladesh J. Bot. 42(2): 265-272, 2013 (December)

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

To determine the optimum pH, the fungus was grown on PDA at 25ºC and varying pH (4-10) for seven days. Initially, the mycelial growth of the fungus in different pH did not vary much but at maturity pH values were found to set profound effect on growth. From day 3 to 5, the fungus showed consistent mycelial growth from pH 6 to 8 but on 6th and 7th day, statistically significant maximum growth was observed at pH 7 and 8, followed by pH 5, 6 and 9 respectively. The optimum pH for the growth and sporulation ranged between 5 and 8. The maximum frequency of spore-length ranged between 25 and 74.9 μ and pH 5 to 10. At pH 4, most of the spore lengths were between 75 and 99.9 μ. Maximum spore length ranged between 150 and 174.9 μ, observed at pH 4 to 6. Though spores with minimum length (1 - 24.9 μ) were common in different pH but the frequency was relatively higher at pH 4 and 10. Karl Pearson’s co-efficient of rank correlation between pH and spore length was –0.297; which indicates the spore length decreases with the increase of pH values. t-test was performed to assess the level of significance. At the 99% confidence level the calculated value of ‘t’ was 3.86, greater than the tabulated value of t (α=0.01) = 2.57. The correlation between pH and spore length was little and negative, but statistically highly significant. Alternately it may be inferred that on PDA medium at 25°C temperature, lower pH values favours increase in spore length of C. cassiicola but reduces spore frequency at per unit area. The fungus showed variable conidial length with pH. C. cassiicola is regarded as a very variable species within the genus. A comparison of C. cassiicola collected on 20 different host plants from various parts of the world has shown a great variation of conidial size for this species. Another study, involving the comparison of genetic differences among different isolates of C. cassiicola, using PCR-based techniques, has shown that there were no detectable genetic differences among such isolates. Thus the phenomenon of producing variable conidial length remained unexplained.

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