TWO additional phytotoxins produced by Bipolaris sp. strain 36, a pathogen of Johnson grass, have been identified as victoxinine [3] and prehelminthosporolactone [4]. We have recently described the isolation and identification of prehelminthosporol [1] and dihydroprehelminthosporol [2] as phytotoxins produced by Bipolaris sp. strain 36 (ATCC 64838), a fungal pathogen of Johnson grass [Sorghum halepense (L.) Pers.] (1). While scaling up production of these phytotoxins we have isolated two minor phytotoxic metabolites identified as victoxinine [3] and prehelminthosporolactone [4]. Culturing of the pathogen and initial purification of the crude extract were carried out as described previously (1). Fractions containing 2 also showed the presence of a white spot against a light blue background on tlc plates treated with phosphomolybdic acid. Purification of the mixture by flash cc yielded each component in pure form. The metabolite that does not react with phosphomolybdic acid was identified as victoxinine [3], a nitrogen-containing terpenoid first isolated from culture filtrates of Helminthosporium victoviae (2), the causal agent of Victoria blight of oats. Victoxinine has been reported from filtrates of Helminthosporium sativum (reclassified as Bipolaris sorokiniana) (3), as well as from pathogenic and non-pathogenic strains of H. victoriae (2,4,5). The structure of victoxinine was first elucidated by chemical correlation with 1 (3). A more recent report on the isolation of victoxinine-6-glycerophosphate from H. victoriae (6) provides ¹H and ¹³C chemical shift data for 3 that are identical to our spectral data.A further new metabolite was obtained as colorless needles after purification by flash cc of a fraction containing 1 as the major component. Both the molecular formula of C₁₅H₂₂O₂ and the spectroscopic data for this new metabolite were found to be identical to those reported for prehelminthosporolactone [4] obtained by oxidation of 1 (1,7,8). While the lactone is a possible product of air oxidation of 1, its presence in culture filtrate at 48 h after inoculation suggests that it is a metabolic product of Bipolaris sp. strain 36. This is the first report of the lactone as a naturally occurring substance.Of the four metabolites mentioned, only victoxinine has been reported previously as having phytotoxic activity. The hydrochloride salt of victoxinine inhibited root growth of H. victoriae -susceptible and -resistant oat plants at 75 μg/ml (2). When tested by the root inhibition assay, victoxinine free base was found to be toxic to cereals in the following order: oats > rye and barley > wheat > sorghum (9). We have evaluated the phytotoxicity of the four metabolites against a number of plants using the leaf spot assay as described (1). The results of this evaluation, listed in Table 1, were determined by considering the time required for the test metabolite to cause a visible lesion and the extent of the lesion. Metabolites 1, 2, and 4 produced lesions on Johnson grass and sorghum resembling those observed in the fungal disease in the field: a reddish brown center surrounded by a black circle at the edge of the drop containing the phytotoxin, with an outer, chlorotic zone extending beyond the drop. In contrast, victoxinine did not provoke the reddening but gave a water-soaked translucent appearance with defined but irregular necrotic boundaries. The red wound response is caused by stress response metabolites of sorghum identified as 3-deoxyanthocyanidins (10). The wound-induced antifungal anthocyanidins have been characterized as phytoalexins of sorghum. The fungal phytotoxins 1, 2, and 4 are elicitors of much stronger reddening than caused by wounding alone, but victoxinine is a toxin which does not elicit this phytoalexin response in sorghum. In corn and bentgrass the effects caused by all four metabolites were similar in appearance: a light brown area surrounded by a chlorotic zone. Sicklepod and morning glory showed the phytotoxic effects of the different metabolites by the presence of necrotic lesions that at high concentrations extended beyond the area under the drop. Dihydroprehelminthosporol is the most toxic metabolite of Bipolaris sp. strain 36 against all of the plants tested except morning glory.