Seven lettuce cultivars were transplanted into a field infested with Fusarium oxysporum f. sp. lactucae (causal agent of Fusarium wilt of lettuce) in August and September of 2014, 2015, and 2016. For moderately susceptible cultivars, 1- or 2-week differences in planting date had a significant effect on severity of Fusarium wilt. In growth chamber experiments, cultivars of moderate susceptibility were maintained in a growth chamber under cool conditions (23/18°C), and a subset of plants was transferred to a warm chamber (33/23°C) for 1 week, at weekly intervals after transplanting. Plants exposed to high temperatures at 2 and 3 weeks after transplanting (WAT) had more severe symptoms of Fusarium wilt than those exposed at 4 WAT. In October 2015, April 2016, August 2016, and August 2017, moderately susceptible cultivars were planted into field soil with an inoculum density gradient. Moderately susceptible cultivars were little affected by Fusarium wilt where inoculum densities of F. oxysporum f. sp. lactucae were <125 per gram of soil, even in warm planting windows. Adjusting planting dates to avoid high temperatures during a critical stage of growth and maintaining low inoculum density in soil can contribute to management of Fusarium wilt in moderately susceptible lettuce cultivars.
Fusarium oxysporum f. sp. lactucae, the cause of Fusarium wilt of lettuce, can survive on crop residue in soil. Persistence of the pathogen over time will be influenced by the rate at which residue decomposes. We evaluated the effect of drying and fragmenting crop residue on the rate of decomposition and survival of F. oxysporum f. sp. lactucae. In a controlled experiment that represented optimal drying conditions, fragmenting and oven-drying infested lettuce taproots at 30°C significantly reduced the frequency of recovery of the pathogen, compared to untreated tissue. However, in a field experiment, drying infested crop residue on the soil surface prior to incorporation did not significantly reduce survival of F. oxysporum f. sp. lactucae after one year. Regardless of treatment, there was not a significant decrease in soil inoculum density between one and 12 months after residue was incorporated. In a greenhouse experiment, fragmenting crop residue prior to incorporation in pathogen-free soil resulted in significantly higher inoculum densities of F. oxysporum f. sp. lactucae after one year. The increase in inoculum levels was associated with a faster rate of residue decomposition, which may be beneficial in the long run but not where lettuce will be replanted within the next year.
Fusarium wilt of lettuce, caused by Fusarium oxysporum f. sp. lactucae, is now found in all major lettuce producing regions in California and Arizona. The population structure of F. oxysporum f. sp. lactucae in California and Arizona was characterized based on somatic compatibility and sequences of the translation elongation factor 1-α gene (EF-1α) and rDNA intergenic spacer region (IGS). In this study, 170 isolates were tested for somatic compatibility based on heterokaryon formation, using complementary nitrate nonutilizing (nit) mutants. Five subgroups (A to E) of somatic compatibility group 0300 were identified. Isolates associated with the same subgroup had a strong complementation reaction, whereas reactions between isolates of different subgroups were weak or delayed. An isolate from the first known infestation of Fusarium wilt of lettuce in California was associated with subgroup A, which predominated among isolates in our collection. Isolates representative of each subgroup were confirmed to be associated with race 1, based on the reaction of differential lettuce cultivars. It is possible that somatic compatibility subgroups B to E of F. oxysporum f. sp. lactucae were derived from subgroup A, as a consequence of somatic mutations affecting compatibility. If so, subgroups of F. oxysporum f. sp. lactucae may represent an intermediate step in divergence that will lead to clearly separable compatibility groups. Sequences of EF-1α and IGS were both identical for 58 isolates of F. oxysporum f. sp. lactucae that represented all somatic compatibility subgroups and locations from which isolates were obtained, indicating that subgroups were derived from the same clonal lineage (VCG 0300).
Industrial hemp (Cannabis sativa) is a newly legal crop in California that is grown for cannabidiol oil, fiber and seed. In August 2019, whole plant decline and root rot were observed affecting <5% of plants in two industrial fields in Fresno County, CA. Symptoms included chlorotic, collapsed foliage, stem vascular discoloration, and root rot with abundant mycelial growth. Stem and root segments (1-2 cm) from three to five diseased plants were agitated in 0.1% tween-20 and soaked in 70% ethanol for 30 s and 1% NaOCl for 2 min. After incubating for 5 to 7 days on 1:10 potato dextrose agar (PDA) amended with tetracycline, Fusarium selective medium (FSM), and PARP (pimaricin + ampicillin + rifampicin + pentachloronitrobenzene [PCNB] agar) medium, white to pale cream aerial mycelium emerged from tissue of all plants on PDA and FSM but not PARP. Isolates cultured on 0.1% potassium chloride agar formed heads of microconidia on long monophialides consistent with the Fusarium solani species complex (FSSC) (Leslie and Summerell 2008). To obtain pure cultures of two isolates (CS529 and CS530), a single-hyphal tip was excised and grown on PDA. DNA was extracted from actively growing mycelium (PrepMan Ultra kit). The translation elongation factor gene (EF-1α) was amplified via PCR using EF1/EF2 primers (O’Donnell et al. 1998). Sequences of the two isolates were identical and deposited under accession number MW892973 in GenBank. The 599 bp sequence was 99.33% identical to FSSC 3 + 4 (Fusarium falciforme) accessions FD_01443_EF-1a based on FUSARIUM-ID BLAST analysis. To evaluate pathogenicity, stems of hemp plants (cv. ‘Berry Blossom’; n=8 plants per isolate) were wounded by penetrating the epidermis in an area about 0.5-cm square by 1-mm deep and 8-inches above the soil line. A 0.5 cm-diameter plug of 7-day old F. falciforme-colonized PDA was placed against the wound. Inoculation sites were loosely wrapped with parafilm for 2 days. A negative control consisted of a sterile PDA plug (n=3). Treatments were arranged in a completely randomized design in a greenhouse. The experiment was conducted once, due to regulatory restrictions at campus facilities. At 61 days post-inoculation, external stem lesions were significantly larger in diameter (P < 0.05; Tukey’s HSD) in plants inoculated with CS529 (8 ± 1 mm) compared to the control (2 ± 0 mm), and larger but not significant for CS530 (6 ± 1 mm). Internal stem lesions (i.e., rot in stele) were observed in plants inoculated with CS529 (9 ± 3 mm); stem rot was very minor in plants treated with CS530 (1 ± 1 mm) and nonexistent for control plants. No other disease symptoms were observed. F. falciforme was isolated from stems of CS529- and C530-inoculated plants. Sequences of re-isolates matched 100% with accession MW892973. These results suggest that F. falciforme causes rot in hemp in California. These studies specifically confirm stem rot abilities; field observations of root rot indicate root rotting abilities, but further tests are needed for confirmation. This is the first report of F. falciforme causing disease in industrial hemp. FSSC was described as causing foot rot in hemp in Italy (Sorrentino et al. 2019), but these isolates belonged to phylogenetic species 5 (F. solani) not F. falciforme. In addition, F. falciforme was reported as causing root rot in hydroponically grown cannabis (Punja and Rodriguez 2018). These studies provide the foundation for development of management tools for hemp disease.
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