Pathogen Growth Inhibition in Ocimum Basilicum (L.), Malus Domestica (Borkh.), and Citrus Limon (L.) by Low-Dose UV-C LED Exposure

Research Article

Ann Agric Crop Sci. 2024; 9(1): 1144.

Pathogen Growth Inhibition in Ocimum Basilicum (L.), Malus Domestica (Borkh.), and Citrus Limon (L.) by Low-Dose UV-C LED Exposure

Paolo Di Lazzaro1*; Giulio Metelli2; Sarah Bollanti1; Antonia Lai3; Marco Montecchi4; Daniele Murra1; Guido Bernabei5; Loretta Bacchetta5

1ENEA, Department FSN-PLAS-PAX, Centro Ricerche Frascati via E. Fermi 45 00044 Frascati, Italy

2University of Tuscia, Dept. DAFNE via S. Camillo de Lellis 01100 Viterbo, Italy

3ENEA, Department FSN-TECFIS-DIM, Centro Ricerche Frascati via E. Fermi 45 00044 Frascati, Italy

4ENEA, Department TE RIN-STSN-ITES, Centro Ricerche Casaccia via Anguillarese, 301, 00123 Santa Maria di Galeria, Italy

5ENEA, Department SSPT-BIOAG-PROBIO, Centro Ricerche Casaccia via Anguillarese, 301, 00123 Santa Maria di Galeria, Italy

*Corresponding author: Paolo Di Lazzaro ENEA, Department FSN-PLAS-PAX, Centro Ricerche Frascati via E. Fermi 45 00044 Frascati, Italy. Tel: +390694005722 Email: paolo.dilazzaro@enea.it

Received: November 25, 2023 Accepted: January 02, 2024 Published: January 09, 2024

Abstract

In order to stimulate hormesis in basil, apple and lemon we used Ultraviolet C-band (UV-C) radiation emitted by an array of Light-Emitting Diodes (LEDs) at low irradiation doses followed by inoculation of the pathogens Botrytis cinerea and Penicillium digitatum. The LEDs are preferable to conventional mercury lamps when robustness and portability are required. Hormetic doses lower than those reported in the literature were used, in order to achieve short irradiation times and thus rapid treatment of crops. The results show that a dose of just 0.3 kJ m-2 released in a time interval of 3 to 14 seconds generates metabolites that inhibit, or dramatically slow, pathogen growth. After 75 days from irradiation immediately followed by inoculation, the fungal development on basil plants affected less than 30% of the epigeal part, compared to 90% in the unirradiated control. In addition, we obtained preliminary results of low-dose hormetic irradiation of lemons and apples. No fungal growth was observed in 75% of irradiated apples 15 days after irradiation and inoculation. Irradiated lemons showed complete inhibition of P. Digitatum growth. One can then infer that preventive irradiation by LED is beneficial to limit crop diseases in both pre-harvest and post-harvest without harming the plant or the environment, as a sustainable alternative to pesticides.

Keywords: Crop protection; Ecological agriculture; Hormesis; Post harvest; Sustainable agriculture; UV-C

Introduction

Overuse of pesticides in agriculture contributes to soil, water, and air pollution, putting the health of operators and consumers at risk. The proposed European target is to reduce the use of chemicals by 50% by 2030 [1]. This goal implies an urgent need to find alternatives to pesticides and copper compounds, the latter of which are used on organic farms and whose toxicity to farmers, birds, mammals, and soil organisms has been demonstrated in long-term assessments.

UV-C radiation (wavelength range of 200-280 nm) appears to be a promising alternative to protect plants and fruits from pathogens without harming the plant or the environment [2-9]. Depending on the dose, the UV-C irradiation generates a germicidal or hormetic response, the latter being an adaptive strategy through which performance is enhanced and mediated. Specifically, exposure to the right dose of UV-C causes stress in plant tissues, which stimulates the biosynthesis of defensive secondary metabolites with antimicrobial and antioxidant activity [3,6,7]. Several works have demonstrated the effectiveness of UV-C irradiation at the wavelength of 254 nm emitted by low-pressure mercury lamps to limit the spread of common pathogens [2,5,7,9]. However, since nearly 140 countries have joined a global agreement (Minamata Convention on Mercury) that is expected to reduce the production and trade of mercury-containing products [10], including mercury lamps, it would be interesting to test other mercury-free UV-C sources possibly with better portability and efficacy. Light-Emitting Diodes (LEDs) are another type of UV-C radiation source with less conversion of electrical power to radiation than lamps, but they are more compact, lightweight, easy to carry, shock resistant, have an immediate turn-on time, and require a smaller and lighter power supply. These characteristics make LEDs preferable to mercury lamps for applications requiring portability, such as field irradiation, without the disposal problems of mercury, which is highly toxic to the environment. Among crop fungal diseases, Botrytis cinerea (Pers. 1794) a necrotic fungus that causes grey mold disease, and Penicillium digitatum Sacc., the causative agent of green mold disease, are among the most dangerous pathogens that infect several species in pre-harvest and remain latent until the post-harvest period, during storage and transport,making their control throughout the supply chain critical [11]. They produce significant economic losses in crop production and storage [12,13]. In particular, B. cinerea, a highly polyphagous necrotrophic pathogen capable of infecting numerous agricultural species, causes significant economic damage to crops on more than 1,400 known hosts in 586 plant genera and 152 botanical families [14]. Similarly, P. digitatum is a major cause of postharvest economic losses, accounting for up to 90% of total damage to citrus, especially in dry areas and subtropical climates [15,16]. Today, the application of chemical synthetic fungicides is the main method of controlling postharvest diseases caused by B. cinerea or P. digitatum [17]. However, there are two main problems with the application of fungicides: 1) toxicological residues of chemical fungicides are harmful to the environment and humans, and 2) the frequency of resistant strains to different categories of fungicides [18] increases crop management costs as well as the difficulty of controlling these pathogens with conventional methods [15].

In this work, it is explored the efficacy of the 277 nm wavelength emitted by an LED array to stimulate hormesis in basil plants and fruits after low-dose irradiation and subsequent exposure to the pathogens B. cinerea or P. digitatum. As case studies it has been chosen: basil plants, one of the main Mediterranean aromatic crops widely used in cooking and medicine; Golden Delicious, the most popular and easily available apple; and typical Italian lemons, of the Femminello cultivar.

Materials and Methods

Plants and Pathogens

Basil plants (Ocimum basilicum (L.), Genovese type) sown in March 2021 in the growth chamber (temperature 25°C; relative humidity 70%) and transplanted (3 plants per pot) at the emission of two true leaves were used. Growing conditions were as follows: 16h photoperiod; the average of PFD 120±10μmol photons m-2 s-1, actinic light. The substrate used for the experiment was the nursery soil with the main characteristics: pH (in H2O)=8; electrical conductivity EC=0.8 dS/m; dry bulk density =220 kg/mc; total porosity =82%. One month after transplanting, the plants were inoculated and irradiated as described in the following.

As for fruits, apples (Malus domestica (Borkh.) Golden Delicious variety) and lemons (Citrus limon (L.) Femminello variety) from organic farming were used.

The fungi were isolated from naturally infected material and subsequently grown in vitro to obtain enough inoculum. B. cinerea (Pers. 1794) was isolated from infected basil plants, while P. digitatum (Pers.) Sacc. 1881 was isolated from infected lemons, see Figure 1. Both fungal pathogens are non-obligatory hosts because once isolated they can be propagated in vitro on artificial culture media and kept in growth chambers.