First Report of Fusarium oxysporum f. sp. niveum Race 3 Causing Wilt of Watermelon in Florida, U.S.A.

Abstract

Fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon) is an important watermelon (Citrullus lanatus) disease in Florida and the southeast. This disease can cause complete yield loss when susceptible cultivars are grown (Egel and Martyn 2013). Disease symptoms include vascular discoloration, especially around crown and upper taproot and withering and wilting leaves followed by death of either a vine or the whole plant. A Fon isolate was collected from Madison County, Florida, in May 2015. It was recovered from infected stem slices that were surface sterilized for 1 min in 0.85% sodium hypochlorite and rinsed with sterile water. The slices were incubated on quarter strength potato dextrose agar (QPDA) until hyphae emerged from the vascular tissue. Conidia from these slices were streaked onto water agar. After 15 h, a germinated conidium was transferred to QPDA to obtain a monoconidial culture and labeled 150525. The isolate was amplified by PCR using EF-1α and IGS (O’Donnell et al. 2009) loci. BLAST analysis of the resulting EF locus sequence (accession no. MF784504) revealed >99% similarity with e-value of zero to F. oxysporum isolate ICMP:20827 (KX534005). Our IGS sequence (MF784503) had a 99% identity to the F. oxysporum accession no. KU128972. Although KU128972 belongs to f. sp. cyclaminis, previous studies had shown F. oxysporum f. sp. are polyphyletic and could cluster with different groups of the F. oxysporum species complex (O’Donnell et al. 2009). Race typing of isolate 150525 was completed using a root dip inoculation method with four differential watermelon cultivars Black Diamond, Charleson Gray, Calhoun Gray, and PI-296341-FR (Martyn and Netzer 1991; Zhou et al. 2010). PI-296341-FR is resistant to race 2 but susceptible to race 3. All other differentials are susceptible to both race 2 and 3. The roots of 2-week-old seedlings were dipped in a conidial suspension (1 × 10⁶ conidia/ml) for 30 s and then planted in transplant flats with potting media. Six seedlings of each differential were challenged with 150525 and a known race 2 identified at the University of Georgia. The flats were kept in a greenhouse with 26°C daytime and 22°C nighttime temperature, and a 14-h photoperiod. Control seedlings were inoculated with water. Seedlings were observed for disease symptoms every week for 1 month. Since there are reports of low-level segregation of PI-296341-FR line (Martyn and Netzer 1991), we conducted four race typing experiments. On all occasions, all differentials inoculated with 150525 were completely wilted with 100% incidence while control seedlings were asymptomatic. The verified race 2 isolate caused PI-296341-FR wilting only once in four experiments with 67% incidence. The pathogen was reisolated from symptomatic plants and confirmed as F. oxysporum using colony and spore morphology (Leslie and Summerell 2006). After confirmation of isolate 150525 as race 3, we tested an additional 26 Fon isolates collected in 2015 using the same techniques mentioned above. Of these, nine isolates from Madison, Levy, and Lee counties of Florida were confirmed as race 3. Currently, commercially available cultivars lack resistance to Fon race 3. Thus, producers in Florida will need to modify their disease management strategies to include delayed planting dates, fumigant and fungicide applications, and novel methods such as grafting for adequate disease control.