Validation of Standard Area Diagrams to Estimate the Severity of Septoria Leaf Spot on Stevia in Paraguay, Mexico, and the United States.

Septoria leaf spot (SLS) affects stevia leaves, reducing their quality. Estimates of SLS severity on different genotypes are made to identify resistance and as a basis to compare management approaches. The use of standard area diagrams (SADs) can improve the accuracy and reliability of severity estimates. In this study, we developed new SADs with six illustrations (0.5, 1, 10, 25, 40, and 75% severity). The SADs were validated by raters with and without experience in estimating SLS. Raters evaluated 40 leaf photos with SLS severities ranging from 0 to 100% without and with the SADs. Agreement (ρc), bias (Cb), precision (r), and intracluster correlation (ρ) coefficients were significantly closer to "true" severity values when the SADs was used by inexperienced (ρc = 0.89; Cb = 0.97; r = 0.90, ρ = 0.81) and experienced (ρc = 0.94; Cb = 0.99; r = 0.95, ρ = 0.91) raters. The SADs were tested under field conditions in Paraguay, Mexico, and the United States, with inexperienced raters assigned to two groups, one SADs trained and the other not trained, that estimated SLS severity three times: first, all raters without SADs and no time limit for the estimates; second, only the SADs-trained group used SADs and no time limit; and third, only the SADs-trained group used SADs, with a time limit of 10 s imposed per specimen assessment. Agreement and reliability of SLS severity estimates significantly improved when raters used the SADs without a time limit. The use of the new SADs improved the accuracy, precision, and reliability of SLS severity estimates, enhancing the uniformity in assessment across different stevia programs.

Anaerobic Soil Disinfestation Reduces Viability of Sclerotinia sclerotiorum and S. minor Sclerotia and Root-Knot Nematodes in Muck Soils.

Experiments were conducted to evaluate potential functional and mechanistic differences in the suppression of Sclerotinia sclerotiorum and S. minor and root-knot nematodes in muck soils by anaerobic soil disinfestation (ASD) using different carbon source amendments. Volatile compounds produced during ASD in muck soil amended with molasses, wheat bran, or mustard greens at 20.2 Mg/ha or a 2% ethanol solution significantly reduced the mycelial growth and number of sclerotia produced by both Sclerotinia spp. compared with the anaerobic control. In amended soils, acetic and butyric acids were detected in concentrations that reduced the viability of sclerotia of both pathogens. Higher concentrations of carbon dioxide were observed in ASD-treated soils, regardless of the amendment, than in the nonamended anaerobic control. Only amendment with wheat bran did not increase the production of methane gas during ASD compared with the controls. Meloidogyne hapla survival was completely suppressed in soils treated with ASD regardless of carbon source. Field trials were conducted in Ohio muck soil to assess survival of sclerotia of both Sclerotinia spp. The viability of sclerotia of both Sclerotinia spp. was significantly reduced in soil subjected to ASD amended with wheat bran (20.2 Mg/ha), molasses (10.1 Mg/ha), or wheat bran (20.2 Mg/ha) plus molasses (10.1 Mg/ha) compared with the controls. A consistent negative correlation between soil reduction and viability of sclerotia of both pathogens was observed. Wheat bran and molasses are both widely available amendments that can be used as ASD carbon sources for the management of soilborne pathogens in muck soils.