Efficacy of different fungicides against Alternaria alternata causing leaf spot of date palm (Phoenix dactylifera L.) under in vitro conditions

Introduction

Date palm (Phoenix dactylifera L.) is an important fruit crop of arid and semi-arid regions and is valued primarily for its fruits as well as for the diverse uses of different plant parts. Like other perennial fruit crops, date palm is affected by a number of fungal diseases. Among these, leaf spot is important because repeated infection can reduce the functional photosynthetic leaf area and adversely influence plant vigour and productivity. The leaf spot symptoms recorded in the supplied study material consisted of small light-brown to darkbrown necrotic specks on leaflets. The spots enlarged and became irregular to elongated, and adjacent lesions could coalesce to form larger blighted areas followed by drying of affected tissues. Such symptoms were associated with Alternaria infection in the investigation. (Arafat et al., 2024) [1] Alternaria alternata is a widely distributed fungal pathogen with a broad host range and is associated with leaf spot diseases on several crops. In the present investigation, the fungus associated with date palm leaf spot was isolated on Potato Dextrose Agar and purified by repeated subculturing. The isolate produced light greyish growth that subsequently became dark olivaceous to black with concentric zonation. Microscopic examination revealed septate, branched and brownish mycelium together with brown, obclavate to muriform conidia possessing transverse and longitudinal septation. Pathogenicity was demonstrated on healthy date palm seedlings followed by re-isolation of the same pathogen. Effective management of leaf spot requires suppression of the pathogen and reduction of disease development. Chemical fungicides are important components of preliminary disease-management studies because in vitro screening can identify compounds with strong inhibitory activity before they are subjected to field validation. Earlier studies cited in the supplied literature have reported variable activity of fungicides against Alternaria spp., including mancozeb, carbendazim, copper oxychloride and other compounds. Studies specifically associated with date palm and Alternaria have also reported substantial activity of selected fungicides under laboratory conditions. (Mansoori, 2012). The efficacy of fungicides can vary with the pathogen isolate, host, fungicide formulation and concentration. Therefore, screening the locally isolated pathogen is useful for identifying treatments that merit further in vestigation. Accordingly, the present study was undertaken to evaluate and compare the in vitro efficacy of six selected fungicides against the isolated A. alternata causing leaf spot of date palm.

Materials and Methods

Experimental site and pathogen

The investigation was carried out during 2025-2026 in the laboratory of the Department of Plant Pathology, Dr. Sharadchandra Pawar College of Agriculture, Baramati. Diseased date palm leaves were collected from the Malegaon, Baramati region. The associated fungus was isolated on Potato Dextrose Agar and purified by repeated subculturing. The isolate was identified as Alternaria alternata on the basis of cultural and microscopic characteristics, and pathogenicity was confirmed on healthy date palm seedlings.

Poisoned food technique

The fungicides were evaluated by the poisoned food technique using Potato Dextrose Agar (PDA) as the basal medium. Appropriate quantities of each fungicide were incorporated into sterilized and cooled PDA. Approximately 20 ml of the treated medium was poured into sterile 90-mm Petri plates. Each plate was inoculated aseptically with a 5-mm mycelial disc cut from an actively growing culture of A. alternata. PDA without fungicide served as the untreated control. Three replications were maintained for each treatment. Plates were incubated at 25 ± 2 °C, and observations were recorded when the untreated control attained full colony growth.

Observation and calculation

Radial mycelial growth was measured in millimetres. The percentage inhibition of mycelial growth over the untreated control was calculated using the formula:

Percentage inhibition = 𝐶 -𝑇 𝐶 × 100

where C represents colony growth in the untreated control and T represents colony growth in the fungicide-treated plate. The methodology in the supplied study material attributes the calculation to Arora and Upadhyay (1978) and Vincent (1927) [2] .

Statistical Analysis

The experiment was analysed under Completely Randomized Design (CRD) following the statistical procedure cited in the supplied study material (Panse et al., 1963) [3] . Percentage observations were transformed by the arcsine method for statistical analysis. Standard error and critical difference at P = 0.05 were used for comparison among treatments.

Results and Discussion

The six fungicides differed markedly in their ability to suppress the mycelial growth of A. alternata. All fungicide treatments reduced colony growth compared with the untreated control, which recorded 90.00 mm colony diameter. The observed reduction in colony diameter corresponded with increased inhibition of mycelial growth. Copper oxy-chloride 50% WP was the most effective treatment against A. alternata, recording the minimum mean colony diameter of 5.00 mm and the maximum mycelial growth inhibition of 94.44%. The high level of inhibition indicates strong suppression of the pathogen under the conditions of the poisoned food assay. Captan 50% WP ranked second, with 17.00 mm colony diameter and 81.11% inhibition.

Propineb 70% WP and Mancozeb 75% WP were the next most effective treatments, recording mean colony diameters of 18.33 and 20.67 mm and corresponding inhibition values of 79.63 and 77.04%, respectively. These treatments therefore produced substantially greater suppression than Chlorothalonil 75% WP and Carbendazim 50% WP. Chlorothalonil 75% WP recorded 30.33 mm mean colony diameter and 66.30% inhibition, whereas Carbendazim 50% WP recorded the highest colony diameter among the fungicide treatments (32.33 mm) and 64.07% inhibition. The untreated control recorded 90.00 mm colony growth and showed no inhibition. Thus, the overall order of efficacy based on inhibition of mycelial growth was Copper oxy-chloride > Captan > Propineb > Mancozeb > Chlorothalonil > Carbendazim.

The superior performance of copper oxy-chloride in the present investigation is in agreement with the broader evidence cited in the supplied literature indicating that copper-based fungicides can suppress Alternaria growth. Garmendia et al. (2025) [4] , as cited in the supplied literature, emphasized that the performance of copper-based products may vary with formulation, dose and disease pressure. The present results also correspond generally with reports cited in the supplied study material in which mancozeb, carbendazim, chlorothalonil and other fungicides reduced Alternaria growth under in vitro conditions. (plate no.1, table no.1, fig. 1). The findings are also comparable with the date-palm and Alternaria studies cited in the supplied literature. Ahmed (2020) [5] , Kadhim et al. (2022) [6] , and Matrood et al. (2021) [7] reported activity of selected fungicides or management treatments against Alternaria associated with date palm. The present isolate, however, showed its highest inhibition with copper oxy-chloride, while the relative efficacy of individual fungicides has varied among studies. Such variation may be associated with differences in pathogen isolate, fungicide formulation, concentration and experimental conditions. The present experiment was conducted under controlled in vitro conditions; therefore, the observed inhibition represents preliminary fungicidal efficacy and should not by itself be interpreted as a fielduse recommendation. Further field validation is necessary to determine disease-control performance, crop safety, practical application schedules and other considerations before any field recommendation is made.

Conclusion

Under the in vitro conditions of the present investigation, all six evaluated fungicides suppressed the mycelial growth of Alternaria alternata compared with the untreated control. Copper oxy-chloride 50% WP was the most effective treatment, recording 94.44% growth inhibition, followed by Captan 50% WP (81.11%), Propineb 70% WP (79.63%) and Mancozeb 75% WP (77.04%). Chlorothalonil 75% WP and Carbendazim 50% WP recorded comparatively lower inhibition.

Copper oxy-chloride, Captan, Propineb and Mancozeb may therefore be considered promising candidates for further field-level evaluation against date palm leaf spot caused by A. alternata.

References

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