Quantitative Phenotyping of Vascular Damage Caused by Fusarium Wilt Disease in Cowpea

Assessment of the severity of Fusarium wilt disease in cowpea and other crops relies mainly on visual rating scales which are prone to errors, which can compromise the reproducibility of the data. Furthermore, the rating scales require considerable practical training and routine experience for reliable assessment. Two objective metrics, stem vascular discoloration length (%VDL) and number of Fusarium necrotic vessels (NFNV), for quantitative measurement of vascular damage incited by Fusarium oxysporum f. sp. tracheiphilum race 4 (Fot4) of cowpea, were compared and their utility as a measure of disease severity and potential usefulness in other crop pathosystems is proposed. The metrics were tested in seven F2 populations and one F2:3 population, segregating for wilt response, and inoculated with race Fot4 at the seedling stage. %VDL and NFNV were highly correlated with plant wilting for all populations (r = 0.51 - 0.93 and 0.52 - 0.94, respectively). Furthermore, the relationships between the variables were linear in all populations (R2 = 0.81 to 0.87 and 0.71 to 0.91), indicating that they can provide accurate and reliable measurement of severity of Fusarium wilt disease. Also, %VDL and NFNV were strongly correlated (r = 0.88 - 0.97) and demonstrated a linear relationship (R2 = 0.69 - 0.94). Analysis of goodness-of-fit in two F2 populations revealed that errors in measurement of vascular discoloration length can result in higher segregation distortion when compared to enumeration of necrotic vessels. However, both metrics were highly effective in accounting for the severity of vascular damage caused by Fusarium wilt disease.