Nematodes are the most damaging pathogens of cotton and one of the most important pathogens of peanut. Weeds can support nematode reproduction and reduce the effectiveness of crop rotation as a management tool. This study documents the relative host status of tropical spiderwort for (1) the reniform nematode and the southern and peanut root-knot nematodes and (2) the fungal pathogen southern stem rot. A reproductive factor (RF) was calculated for each nematode (final number divided by initial number). Galling was estimated (0–10 scale) for the root-knot species. The southern root-knot nematode reproduced well on tropical spiderwort, with a gall rating of 3.1 and an RF of 15.5. The peanut root-knot nematode also reproduced well on tropical spiderwort, with a gall rating of 2.1 and an RF of 7.2. Trials with the reniform nematode were analyzed independently. In the first trial with the reniform nematode, the RF was 2.4 on tropical spiderwort and 1.4 on cotton. In the second trial, the RF was 3.6 on tropical spiderwort and 13.5 on cotton. The severity of symptoms caused by southern stem rot was estimated on a scale of 0 to 10. In the first trial, peanut had a disease severity rating of 4.0 and tropical spiderwort had a rating of 1.4, and the fungus could be seen growing on 40% of the tropical spiderwort plants. In the second trial, peanut had a disease severity rating of 10.0 and tropical spiderwort had a rating of 5.0, but the fungus could be seen growing on all of the tropical spiderwort plants. It appears that tropical spiderwort is a sufficiently good host for some of the primary nematode and fungal pathogens of major crops in the southeastern United States that its presence at typical plant population densities will greatly reduce the pathogen-suppressive effects of crop rotation.
Nomenclature: Tropical spiderwort, Commelina benghalensis L. COMBE; cotton, Gossypium hirsutum L.; peanut, Arachis hypogaea L.; peanut root-knot nematode, Meloidogyne arenaria, reniform nematode, Rotylenchulus reniformis; southern root-knot nematode, Meloidogyne incognita; southern stem rot, Sclerotium rolfsii.