Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 27, 2007
Publication Date: December 3, 2007
Citation: Moran, P.J., Showler, A.T. 2007. Phomopsis amaranthicola and Microsphaeropsis amaranthi symptoms on Amaranthus spp. under South Texas conditions. Plant Disease. 91(12):1638-1646. Interpretive Summary: Palmer amaranth, smooth pigweed, and redroot pigweed interfere with crops. Certain fungi act as ‘bioherbicides’ when they are sprayed onto weeds, leading to dieback of weed infestations. We sprayed two bioherbicides on 18- or 35-day-old Palmer amaranth, smooth pigweed, and redroot pigweed growing in a greenhouse in the Lower Rio Grande Valley of Texas. We found that up to 76% of the leaves and 100% of the stems had dead tissue spots within two weeks. Sprayed redroot and smooth pigweed plants did not grow as well as healthy plants. However, the fungi did not kill any of the three pigweed species, because the plants were too old, and daytime temperature was too high (above 30 C) and relative humidity too low (below 80%), compared to previous studies that killed younger plants. Spraying Palmer amaranth increased the activity of an enzyme involved in resistance to infection. Adding spray ingredients to retain moisture, and increasing the fungal dose, increased spotting but still did not kill the weeds. Infection increased protein content in smooth pigweed, which might benefit pest insects. This research demonstrated the need for further spray development to make bioherbicides useful against mixed pigweed species and ages in this region.
Technical Abstract: The candidate bioherbicides, Phomopsis amaranthicola and Microsphaeropsis amaranthi, were applied singly or in combination to Palmer amaranth (Amaranthus palmeri), smooth pigweed (Amaranthus hybridus), and redroot pigweed (Amaranthus retroflexus). Inoculation induced necrosis on 5 to 10% of leaves and stem lesions on 0 to 30% of A. palmeri plants. Symptoms were 1.5 to 10-fold more common (up to 76% leaf necrosis and 100% stem lesions) in A. hybridus and A. retroflexus, but no mortality occurred. Leaf gain and height gain were reduced by up to 64% and 20%, respectively, in A. hybridus, and symptoms were 1.2 to 4.2 fold greater in mature (35-day-old) than immature (18-day-old) plants. Formulation of inoculum with citrus oil and a polysaccharide increased stem lesion occurrence 72% in A. hybridus. A combined pathogen dose of 1.4 × 105 conidia ml-1 was sufficient to produce symptoms. Increasing the dose to the 107 range did not increase efficacy. Field inoculation with M. amaranthi produced symptom levels similar to greenhouse trials, but did not cause mortality. Inoculation increased protein content 1.9-fold in A. hybridus and peroxidase activity 2.8-fold in A. palmeri, but did not influence free amino acid content. Efficacy was limited by plant age, high temperature and low humidity after inoculation, and resistance in A. palmeri.