Molecular and Environmental Triggering Factors of Pathogenicity of Fusarium oxysporum and F. solani Isolates Involved in the Coffee Corky-Root Disease

Coffee corky-root disease causes serious damages to coffee crop and is linked to combined infection of Fusarium spp. and root-knot nematodes Meloidogyne spp. In this study, 70 Fusarium isolates were collected from both roots of healthy coffee plants and with corky-root disease symptoms. A phylogenetic analysis, and the detection of pathogenicity SIX genes and toxigenicity Fum genes was performed for 59 F. oxysporum and 11 F. solani isolates. Based on the molecular characterization, seven F. oxysporum and three F. solani isolates were assessed for their pathogenicity on coffee seedlings under optimal watering and water stress miming root-knot nematode effect on plants. Our results revealed that a drastic increment of plant colonization capacity and pathogenicity on coffee plants of some Fusarium isolates was caused by water stress. The pathogenicity on coffee of F. solani linked to coffee corky-root disease and the presence of SIX genes in this species were demonstrated for the first time. Our study provides evidence for understanding the pathogenic basis of F. oxysporum and F. solani isolates on coffee and revealed the presence of SIX and Fum genes as one of their pathogenicity-related mechanisms. We also highlight the relevance of chlorophyll, a fluorescence as an early and high-throughput phenotyping tool in Fusarium pathogenicity studies on coffee.

On-farm evaluations of anaerobic soil disinfestation and grafting for management of a widespread soilborne disease complex in protected culture tomato production

Tomato production in Ohio protected culture systems is hindered by a soilborne disease complex consisting of corky root rot (Pyrenochaeta lycopersici), black dot root rot (Colletotrichum coccodes), Verticillium wilt (Verticillium dahliae), and root-knot (Meloidogyne hapla and M. incognita). In a survey of 71 high tunnels, C. coccodes was detected in 90% of high tunnels, while P. lycopersici (46%), V. dahliae (48%) and Meloidogyne spp. (45%) were found in nearly half of high tunnels. Anaerobic soil disinfestation (ASD) with wheat bran (20.2 Mg/ha) plus molasses (10.1 Mg/ha) and grafting onto ‘Maxifort’ or ‘Estamino’ rootstocks were evaluated in high tunnels on five farms. In post-ASD bioassays using trial soils, root and taproot rot severity were significantly reduced following ASD, and root-knot galling was also reduced by ASD. Soilborne pathogenic fungi were isolated less frequently from bioassay plants grown in ASD-treated soils than control soils. Similar results were observed in tomato plants grown in high tunnels. Root rot was significantly reduced by ASD in nearly all trials. Corky root rot severity was highest in non-grafted plants grown in non-treated soils, while the lowest levels of corky root rot were observed in Maxifort-grafted plants. Black dot root rot severity was higher or equivalent in grafted plants compared to non-grafted plants. Root-knot severity was lower in plants grown in ASD-treated soils in high tunnels compared to plants grown in control soils, but grafting did not significantly decrease root-knot severity. However, soil treatment did not significantly impact yield, and grafting led to inconsistent impacts on yield.

Case Investigation and Forensic Evidence for a New Plant Disease: The Case of Lettuce Corky Root

The process of disease diagnosis reminds of the process of solving a crime. This starts with a so-called ‘crime scene investigation’ (CSI) carried out in a highly systematic manner. The CSI is followed by ‘forensic investigation’ in specialized laboratories. The final step in solving a crime is the ‘crime scene reconstruction’ process, which involves systematic elimination of unlikely scenarios and comparison of results from the analysis of physical evidence with eye witness accounts. If more evidence becomes available, an ‘old case may be reactivated’. In this review, the same sequence of activities is followed to solve a plant disease problem using a case study of a disease that was difficult to diagnose, namely the ‘case’ of corky root of lettuce.