Control of seedborne leaf spot disease

The Problem

Solanaceae vegetables including peppers and tomatoes are economically important crops worldwide. In 2020, tomatoes (186 million metric tons) and peppers (36 million metric tons) were among the most produced vegetable crops globally. The USA is one of the top producers of tomatoes and peppers and has a large market for these crops valuing them at 102 million and 32 million, respectively (Value of agricultural product, FAO, 2020). However, plant-pathogenic bacteria like Pseudomonas syringae pathovars and Xanthomonas species pose a serious threat to both the tomato and pepper production industry by significantly affecting the yield and quality of the harvest (Dhakal et al., 2019). These pathogens can spread up to 50-80% of the field under favorable environmental conditions (Miller, 2019), which can cause significant economic losses to tomato and pepper production if these pathogens are left unmanaged. Currently, copper and streptomycin-based antimicrobials are used to mitigate bacterial diseases in pepper and tomatoes (Griffin et al., 2017); however, due to the increase in antimicrobial resistance genes in plant pathogens, the current management techniques are becoming less effective over time (Lamichhane et al., 2018; Le et al., 2020). Specifically, Pseudomonas syringae pv. syringae (Pss), an emerging seed-borne pathogen of peppers is harder to control because it carries multi-drug resistance genes against currently used antimicrobials. Therefore, there is an urgent need to develop novel methods to control these two phytopathogens.

The Solution

Researchers at OSU have identified ten novel compounds (PC1-PC10) that are able 1) to mitigate Pss in infected pepper seedlings, which will assure the yield and quality of the crop, and 2) mitigate Pss in infested pepper seeds, which will facilitate the production and sale of Pss-free pepper seeds. These compounds displayed equivalent or even better antimicrobial efficacy compared to current commercial approaches (200 µM copper and 200 mg/ml streptomycin). A single application of our compounds (at 200 µM using a leaf spraying method at 1-day post-inoculation; DPI) on pepper seedlings significantly reduced Pss disease symptoms (by 89.5%). The disease severity was also associated with significant reduction in Pss load in infected seedlings (by up to 1.93 log CFU/g) compared to the untreated infected seedlings. Similarly, a single application of our compounds (at 200 µM by soaking the seeds for 20 min at 1-day post-infestation) to Pss-infested pepper seeds significantly reduced Pss load (by up to 3.06 log CFU/g).

Furthermore, our compounds can easily be incorporated to already existing seed treatment practices (seed priming, or coating). Our in vivo studies also demonstrated that our compounds (200 µM) displayed no toxic effects on pepper tissues (seeds, seedlings, and fruit), pollinator honeybees, and human Caco-2 cell lines. Therefore, our compounds can be safely used by farmers and will have no adverse effects on the crop and associated pollinators. In addition, we validated in vitro that our compounds were effective against biofilm-protected Pss, copper (< 25 µg/ml) and streptomycin (< 3.125 µg/ml) resistant Pss strains. No spontaneous resistance from Pss to our compounds has been recorded in vitro to date, unlike what we observed for streptomycin using a similar approach.

Preliminary experiments uncovered that our compounds are likely killing bacteria by affecting their cell membrane integrity (Deblais et al., 2018; Kathayat et al., 2018). Drugs targeting the cell membrane put less selective pressure on the pathogen, which reduces the chance of antimicrobial resistance development against the drug (Kathayat et al., 2018), and thus, enhance the antimicrobial potency against the AMR burden. In addition, in vitro data highlighted the potential antimicrobial potency of our compounds against other tomato and pepper pathogens, such as P. syringae pv. tomato from tomatoes and Xanthomonas euvesicatoria from peppers . Thereby, our compounds are an ideal alternative method to manage bacterial pathogens in Solanaceae and assure the yield and quality of the crops.

Our compounds had minimal effects on beneficial plant bacteria such as P. fluorescens and P. protegens, which are used as biocontrol agents to manage Pseudomonas and Xanthomonas. Therefore, our compounds could be combined with these biocontrol agents to enhance the control of bacterial seedborne diseases in tomato and pepper production during the growing season. Furthermore, their unique drug-likeness properties make them ideal candidates for large-scale industrial production and application.


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