Control strategies, screening techniques and breeding for Fusarium oxysporum f. sp. basilici resistance in sweet basil
Creators: name: Homa, Kathryn
Publisher: name: No Publisher Supplied
Year: 2021

description: Sweet basil (Ocimum basilicum, Fam. Lamiaceae) is economically-significant culinary herb grown throughout the world. Fusarium wilt of basil (FOB), caused by Fusarium oxysporum f. sp. basilici, is an economically damaging disease of field- and greenhouse-grown sweet basil that was thought to have been introduced into the U.S. via contaminated commercial seed lots from Italy. The pathogen was first reported in hydroponically-grown basil and in field production in Plymouth County, Massachusetts U.S. in 1990. Growers later observed that when FOB-resistant cultivars, e.g. ‘Nufar’, were exposed to a high concentration of FOB inoculum (e.g., 1.0 x 106 conidia/mL), the cultivar expresses symptoms. Since currently available chemical, biological and cultural control methods are costly, not sustainable, ineffective, and/or challenging to implement, new strategies of FOB control, such as resistant cultivars, are needed.In the 2016 ‘Nufar’ seed source experiment, six commercial seed sources were tested in the greenhouse for their response to a single isolate of FOB at one of three inoculum concentrations (1.0 x 102, 1.0 x 104, 1.0 x 106 conidia/mL). The data indicate that there was a significant (P &lt; 0.0001) interaction of inoculum concentration with cultivar for the Area Under the Disease Progress Curve (AUDPC), final plant height, and mortality. Also, seed source and inoculum concentration influence FOB severity and that host resistance decreases with increasing inoculum concentration. To understand the effect of inoculum concentration (1.0 x 104, 1.0 x 105, 1.0 x 106 conidia/mL), cultivar or line (‘Caesar’, ‘Nufar’, experimental line), and leaf stage (2, 4, 6) on FOB incidence and severity, a split-split plot growth chamber experiment was conducted. At the conclusion of the experiment, to determine if FOB inoculated asymptomatic and symptomatic plants were infected with FOB, whole plant stems of each cultivar or line were randomly sampled from various treatments and blocks, plated on half strength PDA and monitored for presence or absence of FOB. The data indicate that inoculum concentration and leaf stage interact to influence FOB incidence and severity. FOB was recovered from most stems of each cultivar and line, regardless of leaf stage and inoculum concentration, which were rated as healthy throughout the experiment. The data indicate that plants may be infected but remain asymptomatic. Results showed that host plants should be screened for FOB disease response with a broad range of leaf developmental stages and inoculum concentrations. Cultured stem tissue from host plants should also be tested to identify instances of latent asymptomatic infectionsGenetically stable commercial cultivars and advanced experimental breeding lines of basil were evaluated for FOB resistance during the spring and summer of 2015. A unique basil plant with no FOB symptoms, no mortality, and no plant stunting was identified after evaluating germplasm collections of basils and was selected as a parent. That plant was crossed with one of the Rutgers University sweet basil genotypes, used the female parent, as it exhibited classic sweet basil characteristics including large, medium green downward-curved leaves, and a mild to spicy aroma. After several generations of crosses, a new sweet basil was developed with the desired traits and enhanced disease resistance. Following breeding, the DNA content and cytology of the FOB parent lines and each generation of the backcross and selfed lines were assessed for stability.Experiments were conducted from 2018 to 2020 by treating FOB mycelium, inoculated sweet basil seedlings and seeds with various experimental cold plasma treatment devices to understand the effect of cold plasma treatment on FOB incidence and severity. Initial results indicated that while the cold plasma jet treatment did not result in a significant reduction in mean mycelial growth rate or virulence of the pathogen, direct cold plasma jet treatments on seedlings, as well as a cold plasma dielectric barrier discharge treatment on seeds, exhibited varying efficacy. Control of FOB appeared to be strongly dependent on the exposure time to cold plasma. These findings can aid in the standardization of a cold plasma treatment for the commercial basil seed and transplant industry.