Zeyad M. A. Ibrahim

The challenge of managing invasive weed species continues to affect the agricultural industry, presenting ecological, economic, and agronomic hurdles that lead to over 100 billion USD in annual crop losses globally. One such concern is the management of Agrostis stolonifera L., commonly known as creeping bentgrass, particularly due to its ability to form hybrids. This scenario underscores the urgent need for innovative, effective, and environmentally sustainable herbicides, steering the focus toward natural substances as potential candidates. We report here a promising natural lactone, commonly known as menthalactone, which is derived from Mentha piperita L. Its phytotoxic activity was assessed against the monocot, bentgrass, and a dicot, lettuce (Lactuca sativa L.). Menthalactone displayed outstanding activity against bentgrass and was further evaluated for phytotoxic characteristics. The germination of A. stolonifera seeds was significantly inhibited with an IC₅₀ value of 4.9 ± 1.2 µM. In contrast to bentgrass seeds, Lemna pausicostata L. plants were less responsive to menthalactone treatment, shown by an IC₅₀ of 293.4 ± 70.6 µM. Both species are monocots, and the results suggest that menthalactone might have effects on seed germination but not on the metabolism of green tissues. The susceptibility of three common, obnoxious weed species, i.e., ryegrass (Lilium perenne), barnyard grass (Echinochloa crus-galli (L.) P. Beauv.), and crabgrass (Digitaria sanguinalis (L.) Scop.), to menthalactone was assessed. Menthalactone at 1000 µM completely inhibited the germination of all three species of grasses, while 330 µM inhibited germination by less than 50%. The post-emergence application of menthalactone at 1% did not produce a significant inhibitory effect against ryegrass, barnyard grass, or crabgrass.

The U.S. Department of Agriculture (USDA) has established research programs to fight the phytopathogen Colletotrichum fragariae and the invasive red imported fire ant, Solenopsis invicta. C. fragariae is known to cause anthracnose disease in fruits and vegetables, while S. invicta is known for its aggressive behavior and painful stings and for being the cause of significant damage to crops, as well as harm to humans and animals. Many plants have been studied for potential activity against C. fragariae and S. invicta. Among the studied plants, Houttuynia cordata Thunb has been shown to contain 2-undecanone, which h is known for its antifungal activity against Colletotrichum gloesporioides. Based on the mean amount of sand removed, 2-undecanone showed significant repellency at 62.5 µg/g, similar to DEET (N,N-diethyl-meta-toluamide), against S. invicta. The 2-Undecanone with an LC₅₀ of 44.59 µg/g showed toxicity against S. invicta workers. However, neither H. cordata extract nor 2-undecanone had shown activity against C. fragariae despite their known activity against C. gloesporioides, which in turn motivates us in repositioning 2-undecanone as a selected candidate for a Claisen–Schmidt condensation that enables access to several analogs (2a–f). Among the prepared analogs, (E)-1-(3-methylbenzo[b]thiophen-2-yl)dodec-1-en-3-one (2b) and (E)-1-(5-bromothiophen-2-yl)dodec-1-en-3-one (2f) showed promising activity against C. fragariae, revealing a distinctive structural activity relationship (SAR). The generated analogs revealed a clear regioselectivity pattern through forming the C=C alkene bond at position C-1. These data open the window for further lead optimization and product development in the context of managing C. fragariae and S. invicta.