Pathogenicity and virulence of Alternaria alternata and Fusarium oxysporum isolates from wild sunflower (Helianthus annuus) on cultivated sunflower in northern Tamaulipas, Mexico

Background/Objective. Wild sunflower or polocote (Helianthus annuus) is a widespread weed in northern Tamaulipas, Mexico, where it coexists with cereal-based agricultural systems. Due to its close genetic relationship with cultivated sunflower, it may serve as a potential source of inoculum of phytopathogenic microorganisms of epidemiological importance. The objective of this study was to determine the pathogenic capacity of Alternaria alternata and Fusarium oxysporum isolates obtained from polocote and to evaluate the effect of the phenological stage at infection on sunflower development. Materials and Methods. The experiment was conducted in pots under open-field nursery conditions at the Río Bravo Experimental Station, Tamaulipas, Mexico (INIFAP). Previously characterized isolates of A. alternata and F. oxysporum obtained from polocote seed populations were used. Inoculation was performed on plants of the sunflower hybrid RRC 2205 at the V4 and V6 phenological stages using suspensions adjusted to 1 × 10⁶ conidia mL⁻¹. Plant height, head diameter, total dry weight, head dry weight, disease incidence, and disease severity were evaluated. Data were analyzed using analysis of variance (ANOVA) and Tukey’s mean comparison test (α = 0.05). Results. Both pathogens successfully infected cultivated sunflower and produced characteristic symptoms of foliar wilt, necrosis, and stem rot. Disease incidence ranged from 92 to 100% among inoculated treatments. Statistical analyses revealed highly significant treatment effects for all variables evaluated (p ≤ 0.0001). Inoculations performed at the V4 stage caused greater reductions in plant height (43.8 cm vs. 88.6 cm at V6), head diameter (3.4 cm vs. 5.0 cm), and biomass accumulation than those performed at the V6 stage. A. alternata produced the highest levels of foliar disease severity (72.2 %), whereas F. oxysporum had a greater impact on vegetative and reproductive growth. Reisolation confirmed Koch’s postulates. Conclusion. A. alternata and F. oxysporum associated with polocote possess the ability to infect cultivated sunflower (incidence 92 to 100%) and significantly reduce plant development (75-109 cm). These results indicate that wild populations of H. annuus present in northern Tamaulipas constitute an important source of inoculum and alternate host with direct implications for commercial sunflower production.