Crop losses and quality degradation caused by fungal plant diseases are major constraints to food security globally. Botrytis cinerea and Alternaria alternata are the two most important fungal pathogens that can cause severe pre- and post-harvest losses in strawberry and grape industry. Use of fungicides in management of these diseases are inevitable till today due to lack of viable, environmentally friendly alternatives. Series of experiments showed the potentials and limitations of optical radiation in management of these important fungal diseases as an alternative to synthetic fungicides. Continuous exploration of additional tactics to strengthen optical based management strategy leads to exploring the potential of photosensitizers against these pathogens. Photosensitizers could be either natural or synthetic compounds and its derivatives, which could be potentially used as sustainable eco-friendly approach to photodynamically inactivate the plant pathogens.
For this approach, a pool of potential photosensitizer candidates was screened ranging from natural compounds of plant sources to food grade additives and dyes. These non-toxic photoactive compounds were chosen based on their non-interference towards plant growth and development while exhibiting no toxicity. Twelve candidates of photosensitizing compounds were commercially procured. The compounds were initially screened and categorized based on their absorption spectra ranging from UV to red wavelengths. They are, i) Three coumarin-based compounds in the UV range, ii) four compounds in the blue wavelength range (460nm), which includes two natural plant-sourced and two porphyrin-based compounds, iii) two erythrosine-based dyes in the green wavelength range (560nm), and iv) three phenothiazine dyes in the red wavelength range (660nm). All these compounds were screened for their photosensitizing ability against gray mold (B. cinerea) in vitro. The conidial suspensions were mixed with photosensitizers and irradiated for minimum 30 minutes under blue, green, and red light, and for 5 minutes under UV. The conidial suspension mixtures were then drop inoculated to check the colony growth and intensity compared with control (without photosensitizer and light treated conidial suspensions). Subsequently, germination percentage was assessed, 6 h after treatment, from the same experimental setup under the light microscope. From these results, i) curcumin from the blue wavelength range, ii) rose bengal dye from the green wavelength range, and iii) new methylene blue, a phenothiazine dye, from the red wavelength absorption range have shown strong photosensitizing ability in suppressing the pathogen under in vitro conditions. Additional screening of these compounds will be conducted at plant level to check the disease control efficiency and phytotoxicity.
Cold storage after harvest is optimal for fresh fruits to avoid quality deterioration, however, some postharvest diseases, such as gray mold, are cold-adaptive and able to cause losses to the cold-stored fruits. The study evaluated the regulatory effects of light on the fungal growth in vitro and gray mold development on table grapes at either ambient (22°C) or cold (4°C) temperatures. None of the tested light conditions, including white, blue, and red, caused remarkable effects on spore germination or colony growth rates of B. cinerea compared with darkness when they were tested at 22°C. Once the fungal cultures were kept at 4°C, blue light significantly retarded spore germination and hyphal expansion in contrast to other light conditions and darkness. Infection assay on table grapes demonstrated that blue light caused no and suppressive effects at 22°C and 4°C, respectively, on gray mold severity. It was also found that the peroxisome organelle density, reactive oxygen species (ROS) were enhanced upon blue light under cold condition. Gene expression study further revealed that the genes related to ribosome, cell cycle, DNA replication, citrate cycle, proteasome, and phagosome were suppressed by blue light at 4°C. Consequently, blue light illumination under cold condition could result in oxidative stress to the B. cinerea cells, thus retard this cold adaptive pathogen from developing unexpected deterioration on the stored fruits. Future study will test whether synergistic or additive antimicrobial effects would be achieved via applying the photosensitizers from the blue light range under cold and blue light conditions.
Strawberry and grape are the two most important fruit crops grown worldwide for fresh and processed market. Botrytis cinerea and Alternaria alternata are the two most important fungal pathogens that can cause severe pre and post-harvest losses in this industry. At present, intensive use of fungicides and preservatives are inevitable in minimizing these losses. This endanger the UN sustainable development goal of ensuring food security by providing safe food. In this project, we will study the possible replacement of fungicides and preservatives by environmentally friendly optical radiation based strategies. While expanding the potential of optical radiation based strategies to another most important fungal pathogen Alternaria alternata, we will strengthen its practical application efficiency by exploring the i) possible role of melanin in UV screening and optical tolerance of Botrytis species, 2) storage temperature and its interaction with optical radiation in disease management efficiency, 3) selection and optimization of photosensitizer that can be exited with previously optimized optical wavelength range against powdery mildew management (250 nm-280 nm, 550 nm-660 nm), 4) effect of optical treatment on accumulation of secondary metabolites (anthocyanin, flavonols and toxins) and its potential impact on human health.
