The fungus, Alternaria sonchi is considered to be a potential agent for the biocontrol of perennial sowthistle (Sonchus arvensis). A new chlorinated xanthone, methyl 8-hydroxy-3-methyl-4-chloro-9-oxo-9H-xanthene-1-carboxylate (1) and a new benzophenone derivative, 5-chloromoniliphenone (2), were isolated together with eleven structurally related compounds (3–13) from the solid culture of the fungus, which is used for the production of bioherbicidal inoculum of A. sonchi. Their structures were determined by spectroscopic (mostly by NMR and MS) methods. Alternethanoxins A and B, which were reported in A. sonchi earlier, were re-identified as moniliphenone and pinselin, respectively. The isolated compounds were tested for phytotoxic, antimicrobial, insecticidal, cytotoxic and esterase-inhibition activities. They did not demonstrate high phytotoxicity (lesions up to 2.5 mm in diameter/length at a concentration of 2 mg/mL) when tested on leaf disks/segments of perennial sowthistle (Sonchus arvensis) and couch grass (Elytrigia repens). They did not possess acute toxicity to Paramecium caudatum, and showed moderate to low cytotoxicity (IC₅₀ > 25 µg/mL) for U937 and K562 tumor cell lines. However, chloromonilicin and methyl 3,8-dihydroxy-6-methyl-4-chloro-9-oxo-9H-xanthene-1-carboxylate (4) were shown to have antimicrobial properties with MIC 0.5–5 µg/disc. Compound 4 and chloromonilinic acid B were found to have contact insecticidal activity to wheat aphid (Schizaphis graminum) at 1 mg/mL. Compounds 2 and methyl 3,8-dihydroxy-6-methyl-9-oxo-9H-xanthene-1-carboxylate displayed selective carboxylesterase inhibition activity at concentration of 100 µg/mL. Therefore, the waste solid substrate for production of A. sonchi spores can be re-utilized for the isolation of a number of valuable natural products.

The study of fungal antibiotics in their competitive interactions with arthropods may lead to the development of novel biorational insecticides. Extracts of Alternaria tenuissima MFP253011 obtained using various methods showed a wide range of biological activities, including entomotoxic properties. Analysis of their composition and bioactivity allowed us to reveal several known mycotoxins and unidentified compounds that may be involved in the entomotoxic activity of the extracts. Among them, tenuazonic acid (TeA), which was the major component of the A. tenuissima extracts, was found the most likely to have larvicidal activity against Galleria mellonella. In the intrahaemocoel injection bioassay, TeA was toxic to G. mellonella and of Zophobas morio with an LT₅₀ of 6 and 2 days, respectively, at the level of 50 µg/larva. Administered orally, TeA inhibited the growth of G. mellonella larvae and caused mortality of Acheta domesticus adults (LT₅₀ 7 days) at a concentration of 250 µg/g of feed. TeA showed weak contact intestinal activity against the two phytophages, Tetranychus urticae and Schizaphis graminum, causing 15% and 27% mortality at a concentration of 1 mg/mL, respectively. TeA was cytotoxic to the Sf9 cell line (IC₅₀ 25 µg/mL). Thus, model insects such as G. mellonella could be used for further toxicological characterization of TeA.

The use of many fungal phytotoxins as natural herbicides is still limited because they cannot penetrate leaf cuticle without injury and a little is known on their selectivity. In order to assess the herbicidal potential of phytotoxic 10-membered lactones (stagonolide A, stagonolide K, and herbarumin I), the selection of adjuvants, the evaluation of selectivity of the toxins and the efficacy of their formulations were performed. Among four adjuvants tested, Hasten™ (0.1%, v/v) increased phytotoxic activity of all the toxins assayed on non-punctured leaf discs of Sonchus arvensis. When assayed on intact leaf fragments of 18 plants species, 10 species were low to moderately sensitive to stagonolide A, while just five and three species were sensitive to stagonolide K and herbarumin I, respectively. Both leaf damage or addition of Hasten™ (0.1%) to the formulations of the compounds considerably increased or altered the sensitivity of plants to the toxins. Stagonolide A was shown to be non-selective phytotoxin. The selectivity profile of stagonolide K and herbarumin I depended on the leaf wounding or the adjuvant addition. Stagonolide A and herbarumin I formulated in 0.5% Hasten™ showed considerable herbicidal effect on S. arvensis aerial shoots. This study supported the potential of the oil-based adjuvant Hasten™ to increase the herbicidal efficacy of natural phytotoxins.

Ten-membered lactones (TMLs) are an interesting and diverse group of natural polyketides that are abundant in fungi and, to a lesser extent, in bacteria, marine organisms, and insects.

Ten-membered lactones (nonenolides) demonstrate phytotoxic, antimicrobial, and fungicidal activity promising for the development of natural product-derived pesticides. The fungus Stagonospora cirsii is able to produce phytotoxic stagonolides A (1), J (2), K (3) and herbarumin I (4) with high yield. The aim of this study was to create a set of structurally related nonenolides and to reveal the structural features that affect their biological activity. Stagonolide A (1) and C-7 oxidized stagonolide K (11) showed the highest phytotoxicity in leaf puncture assay and agar seedlings assay. The oxidation of C-7 hydroxyl group (as in 1, acetylstagonolide A (10) and (11) led to the manifestation of toxicity to microalgae, Bacillus subtilis and Sf9 cells regardless of the configuration of C-9 propyl chains (R in 1 and 10, S in 11). C-7 non-oxidized nonenolides displayed none or little non-target activity. Notably, 7S compounds were more phytotoxic than their 7R analogues. Due to the high inhibitory activity against seedling growth and the lack of side toxicity, mono- and bis(acetyl)- derivatives of herbarumin I were shown to be potent for the development of pre-emergent herbicides. The identified structural features can be used for the rational design of new herbicides.