|DIAZ, JOSUE - California State University|
|JORGE, GARCIA - California State University|
|LARA, CELESTE - California State University|
|HUTMACHER, ROBERT - University Of California|
|NICHOLS, ROBERT - Cotton, Inc|
|ELLIS, MARGARET - California State University|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2020
Publication Date: N/A
Interpretive Summary: The fungus that causes Fusarium wilt (FOV) is an important and prevalent wilt pathogen known to cause severe production losses across all US cotton growing regions. In California, the soil-borne FOV fungus-type 4 or race 4 (FOV4) was first identified in the state in 2001 and has been an expanding and recurring threat. More recently, FOV4 was reported in Texas in 2017 and New Mexico in 2019. This study aimed to identify current fungus-types or isolates of FOV from infected plants to examine their diversity and aggressiveness during the seedling and wilt stages of disease development. In addition, three different FOV artificial-inoculation assays were examined under control or greenhouse conditions to measure FOV aggressiveness during the seedling stage. A total of 181 isolates of Fusarium were collected from 2017 to 2019 across 13 locations in the San Joaquin Valley of California and 19 isolates from one location in the lower Rio Grande Valley near El Paso Texas during 2018. Analyses of molecular characterization of FOV isolates suggests that there has been a FOV4 population shift in some fields due to selective pressure towards this fungus-type. In California, not one but two unique isolates of FOV4 are common in infested fields. This shift might be the reason why FOV4 has been able to overcome some of the resistance in current commercial Pima varieties. Further, our results suggest the possibility that selection pressure might favor certain FOV4 types in certain production field locations or environmental conditions. Inoculation assays demonstrated the capability of all tested isolates to produce seedling and/or wilt symptoms on cotton. The FOV isolate aggressiveness varied within and across inoculation assays, suggesting that these assays may be used as screening methods, along with field evaluations, for developing cotton varieties with resistance to FOV. This research provides critical and important information for private and public plant breeders as they continue to develop FOV resistant varieties, as there may be a difference in how FOV4 types interact with the plant host.
Technical Abstract: Fusarium oxysporum f. sp. vasinfectum (FOV) race 4, can be the cause of both seedling and wilt symptoms to cotton (Gossypium spp.). FOV race 4 is an expanding and recurring threat to cotton production in California since it was first identified in the state in 2001 and more recently it was reported in Texas in 2017 and New Mexico in 2019. This study aimed to genotype current isolates of FOV from California and examined representative isolates for aggressiveness during the seedling and wilt stages of disease development. A total of 181 isolates of Fusarium were collected from 2017 to 2019 across 13 locations in the San Joaquin Valley of California and 19 isolates from one location in the lower Valley of El Paso Texas during 2018. Isolates were identified to species using DNA sequencing of the translation elongation factor gene (EF-1') and genotyped using FOV race 4 specific primers. From California, 171 isolates were identified as FOV race 4 with 112 and 59 identified as the T and N genotypes, respectively. Sixteen isolates from Texas were identified as the FOV race 4 MT genotype. Eight isolates from California were identified as FOV race 3 and two isolates from California and two from Texas were identified as F. solani. Pathogenicity for representative isolates was tested using three different inoculation assays, a rolled towel, FOV infested-oat seed, and a root dip inoculation assay to test the isolates abilities to produce seedling and/or wilt symptoms on cotton. All isolates that were tested were capable of producing seedling and/or wilt symptoms on cotton, however isolate aggressiveness varied within and across inoculation assays. The results from the pathogenicity screenings demonstrates the abilities of these different inoculation methods for the evaluation of FOV isolate aggressiveness and for use as potential screening methods for cotton germplasm for resistance to FOV. In addition, this research provide critical and important information for private and public breeders as they continue to develop FOV4 resistant cultivars as they may be a difference in how FOV4 genotypes interact with the host plant.