Fusarium oxysporum f. sp. apii race 4 threatening celery production in South Florida.

Fusarium wilt, caused by Fusarium oxysporum f. sp. apii (Foa), constitute a vascular disease affecting celery. This soil-borne pathogen is classified into four distinct pathogenic races: 1, 2, 3, and 4. Notably, race 4 emerges as the most virulent, representing the latest evolutionary development of this pathogen, which was first reported in 2013 in California. In 2022, celery plants in South Florida exhibited typical Fusarium wilt symptoms, with the disease reaching a 100% incidence and causing yield losses ranging from 20% to 100%. Given the significance of celery as a vegetable crop and the severity of this outbreak, the primary objective of this study was to identify and characterize the causal agent of Fusarium wilt in South Florida. The second goal aimed to test the pathogenicity and virulence of the Fusarium isolates from Florida on celery and parsley plants. Using race-specific primers and dual-loci phylogenetic analyses, the isolates surveyed in this study were identified as Foa race 4. Pathogenicity assays in the greenhouse showed that the Foa race 4 isolate from celery induced disease not only on the two celery cultivars (Duda 30 and Duda 71) but also on two commonly cultivated parsley varieties (Curly and Italian). Our study also revealed that Foa race 4 significantly (P < 0.05) affected plant health attributes in all cultivars, including plant height, total plant weight, and root weight. Interestingly, the pathogen exhibited higher (P < 0.0001) virulence on parsley than celery based on vascular discoloration. These findings strongly indicate the urgency of comprehending and managing Fusarium wilt on celery and related crops. Furthermore, the ability of Foa race 4 to affect different plant species highlights a potential threat to agricultural production, emphasizing the need for proactive measures to mitigate the impact of this virulent pathogen.

Detached Maize Sheaths for Live-Cell Imaging of Infection by Fungal Foliar Maize Pathogens.

We have optimized a protocol to inoculate maize leaf sheaths with hemibiotrophic and necrotrophic foliar pathogenic fungi. The method is modified from one originally applied to rice leaf sheaths and allows direct microscopic observation of fungal growth and development in living plant cells. Leaf sheaths collected from maize seedlings with two fully emerged leaf collars are inoculated with 20 µL drops of 5 x 105 spores/mL fungal spore suspensions and incubated in humidity chambers at 23 °C under continuous fluorescent light. After 24-72 h, excess tissue is removed with a razor blade to leave a single layer of epidermal cells, an optically clear sample that can be imaged directly without the necessity for chemical fixation or clearing. Plant and fungal cells remain alive for the duration of the experiment and interactions can be visualized in real-time. Sheaths can be stained or subjected to plasmolysis to study the developmental cytology and viability of host and pathogen cells during infection and colonization. Fungal strains transformed to express fluorescent proteins can be inoculated or co-inoculated on the sheaths for increased resolution and to facilitate the evaluation of competitive or synergistic interactions. Fungal strains expressing fluorescent fusion proteins can be used to track and quantify the production and targeting of these individual proteins in planta. Inoculated sheath tissues can be extracted to characterize nucleic acids, proteins, or metabolites. The use of these sheath assays has greatly advanced the detailed studies of the mechanisms of fungal pathogenicity in maize and also of fungal protein effectors and secondary metabolites contributing to pathogenicity.

Widespread QoI Fungicide Resistance Revealed Among Corynespora cassiicola Tomato Isolates in Florida.

Target spot of tomato caused by Corynespora cassiicola is one of the most economically destructive diseases of tomato in Florida. A collection of 123 isolates from eight counties in Florida were evaluated for sensitivity to azoxystrobin and fenamidone based on mycelial growth inhibition (MGI), spore germination (SG), detached leaflet assays (DLAs), and sequence-based analysis of the cytochrome b gene (cytb). Cleavage of cytb by restriction enzyme (Fnu4HI) revealed the presence of a mutation conferring a glycine (G) to alanine (A) mutation at amino acid position 143 (G143A) in approximately 90% of the population, correlating with quinone outside inhibitor (QoI) resistance based on MGI (<40% at 5 µg/ml), SG (<50% at 1 and 10 µg/ml), and DLA (<10% severity reduction). The mutation conferring a phenylalanine (F) to leucine (L) substitution at position 129 (F129L) was confirmed in moderately resistant isolates (#9, #19, and #74) based on MGI (40 to 50% at 5 µg/ml), SG (<50% at 1 µg/ml and >50% at 10 µg/ml), and DLA (>10% and <43% severity reduction) for both QoI fungicides, whereas sensitive isolates (#1, #4, #7, #28, #29, #46, #61, #74, #75, #76, #91, #95, and #118) based on MGI (>50% at 5 µg/ml), SG (>50% at 1 µg/ml and 10 µg/ml), and DLA (>50% severity reduction) correlated to non-mutation-containing isolates or those with a silent mutation. This study indicates that QoI resistance among C. cassiicola isolates from tomato is widespread in Florida and validates rapid screening methods using MGI or molecular assays to identify resistant isolates in future studies.

Morphological and Molecular Identification of Two Florida Populations of Foliar Nematodes (Aphelenchoides spp.) Isolated From Strawberry With the Description of Aphelenchoides pseudogoodeyi sp. n. (Nematoda: Aphelenchoididae) and Notes on Their Bionomics.

Two Florida populations of foliar nematodes were collected from strawberries originating from Cashiers, North Carolina (USA) located west from Willard, the type locality of Aphelenchoides besseyi. Both nematodes were cultured on Monilinia fructicola and identified using morphological characteristics and molecular assays as Aphelenchoides besseyi and Aphelenchoides pseudogoodeyi sp. n., a herein described new species related to Aphelenchoides goodeyi belonging to the Group of Aphelenchoides exhibiting stellate tails. The morphological and biological characters of Florida A. besseyi fit those of the original description of this species. A. pseudogoodeyi sp. n., which was initially misidentified as Aphelenchoides fujianensis, differed from the type population of the latter species from China because it was without males, and females lacked a functional spermatheca, whereas type A. fujianensis is an amphimictic species. Phylogenetic analyses using near full-length 18S ribosomal RNA (rRNA), the D2-D3 expansion fragments of 28S rRNA, and partial COI gene sequences indicated that A. besseyi is a species complex. A. pseudogoodeyi sp. n. grouped in different clades from those of the type A. fujianensis, instead merging with populations identified of 'A. fujianensis' from Brazil and other countries, suggesting that the latter are conspecific and incorrectly identified. The Florida A. besseyi infected strawberry and gerbera daisy, but not soybean and alfalfa. A. pseudogoodeyi sp. n. is mainly mycetophagous. Localized inoculation of 300 specimens applied with filter paper adhering to the blade of the soybean leaves resulted in nematode penetration into the mesophyll with subsequent development of lesions limited to the inoculated area of the blade.

Characterization of Colletotrichum Species Causing Anthracnose of Pomegranate in the Southeastern United States.

Anthracnose fruit rot and leaf blight caused by Colletotrichum species are important diseases of pomegranate in the southeastern United States. In this study, 26 isolates from pomegranate were identified based on pathological and molecular characterization. Isolates were identified to species based on multilocus sequence analysis with the internal transcribed spacer region, glyceraldehyde-3-phosphate dehydrogenase, ß-tubulin, and chitin synthase genomic genes. Pomegranate isolates grouped within the C. acutatum and C. gloeosporioides species complexes, with more than 73% belonging to the latter group. Three species were identified within the C. acutatum species complex (C. nymphaeae [n = 5], C. fioriniae [n = 1], and C. simmondsii [n = 1]), and three other species were identified within the C. gloeosporioides species complex (C. theobromicola [n = 11], C. siamense [n = 6], and C. gloeosporioides [n = 2]). Inoculations of pomegranate fruit showed that isolates from the C. acutatum species complex were more aggressive than isolates from the C. gloeosporioides species complex. Interestingly, opposite results were observed when leaves of rooted pomegranate cuttings were inoculated. In addition, Colletotrichum isolates from pomegranate, strawberry, blueberry, mango, and citrus were cross-pathogenic when inoculated to fruit. This is the first study identifying six different species of Colletotrichum causing pomegranate leaf blight and fruit anthracnose in the southeastern United States and the potential cross-pathogenic capability of pomegranate isolates to other commercially important crops.