January 19, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of Fusarium oxysporum f. sp. niveum Race 2 as causal agent of fusarium wilt of watermelon in Indiana
D. S. Egel, Purdue University, West Lafayette, IN; R. Harikrishnan, North Dakota State University, Fargo, ND; and R. Martyn, Purdue University, West Lafayette, IN. Plant Dis. 89:108, 2005; published on-line as DOI: 10.1094/PD-89-0108A, 2005. Accepted for publication 17 Oct 2004.

_Fusarium oxysporum_ f. sp. _niveum_ race 1 is uniformly distributed throughout watermelon (_Citrullus lanatus_ (Thunb.) Matsum. & Nakai) growing regions, but _F. oxysporum_ f. sp. _niveum_ race 2 has a limited known distribution in the United States (Texas, Florida, Oklahoma, Maryland, and Delaware) (3,4).

Since the spring of 2001, commercial watermelon fields in Knox and Gibson counties in southwestern Indiana have been observed with symptoms of one-sided wilt and vascular discoloration typical of Fusarium wilt. Race 2
of _F. oxysporum_ f. sp. _niveum_ was suspected as the casual agent, since the diseased watermelon cultivars are considered resistant to races 0 and 1.

2 isolates of _F. oxysporum_ obtained from wilted watermelon plants in 2 different commercial fields and one isolate obtained from a wilted seedling in a transplant house were compared for pathogenicity in a greenhouse
assay. Known isolates of _F. oxysporum_ f. sp. _niveum_ races 0, 1, and 2 were obtained from Don Hopkins (University of Florida, Apopka), Kate Everts (University of Maryland/University of Delaware, Salisbury, MD), and Ray Martyn (Purdue University, West Lafayette, IN) respectively, and were used for comparison.

All isolates were grown in shake cultures in a mineral salts liquid medium (1). After 72 hr, the predominately microconidal suspensions were filtered through cheesecloth and adjusted to 100 000 conidia/ml with the aid of a
hemacytometer. A concentration of 100 000 condia/ml was shown previously to cause the desired disease reaction in the standard cultivars. Seedlings of the differential cvs, Black Diamond (universal susceptible), Charleston Gray (race 0 resistant), and Calhoun Gray (race 0 and 1 resistant) were grown in a 1:1, (v:v) sand/vermiculite mixture to the 1st true-leaf stage, after which the plants were uprooted and the roots carefully washed prior to root dip inoculation.

Subsequent to inoculation, seedlings were planted in a sand/vermiculite/peat mixture (4:1:1, [v:v:v]) with 4 seedlings to a 15-cm-diameter pot. The experimental design was a randomized complete block with 5 replications. 2 isolates from the commercial field plants caused an average of 100 percent wilt on cv. Black Diamond, 95 percent wilt on cv. Charleston Gray, and 80 percent wilt on cv. Calhoun Gray, resulting in a
designation of race 2. The isolate from a commercial transplant house resulted in 100, 60, and 15 percent wilt, respectively, on the 3 standard cultivars resulting in a race 1 designation.

The presence of _F. oxysporum_ f. sp. _niveum_ race 2 in Indiana is significant, because Indiana currently ranks 5th in the United States in watermelon production, and there are no commercially available cultivars that possess resistance to race 2.

To our knowledge, this is the 1st report of _F. oxysporum_ f. sp. _niveum_ race 2 in Indiana and the 1st report of race 2 from the Midwest region of the United States. Race 2, 1st described from the United States in 1985 (2), has now been confirmed in 6 states.

References:
(1) R. Esposito and A. Fletcher. Arch. Biochem. Biophys. 93:369, 1961.
(2) R. Martyn, Plant Dis. 69:1007, 1985.
(3) R. Martyn, Plant Dis. 71:233, 1987.
(4) X. Zhou and K. Everts. Plant Dis. 87:692, 2003.

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[Fusarium wilt (FW) is the most common soilborne disease of watermelon, occurring in Texas, Oklahoma, Florida, Maryland, Delaware and, with this report, Indiana. It has also been reported from Greece, Turkey, Spain and
Cyprus. FW threatens watermelon production in the Northeast region of the USA, where triploid (seedless) watermelons are grown on nearly 50 percent of the acreage. Most triploid cultivars are highly susceptible to FW.
Control of FW is difficult, because once the pathogen builds up in the soil, no effective rotation period will ensure a healthy crop. Host resistance to FW of watermelon is expressed in cultivars as a continuum from highly resistant to susceptible. However, even cultivars rated as resistant may sustain significant wilt when grown in highly infested
fields. Soil fumigants are used in combination with host resistance to reduce FW in Maryland and Delaware. A 5-6 year rotation with non-cucurbits, also recommended for disease management, is not feasible in the mid-Atlantic region, where availability of fumigated land is limited. Use of multiple tactics, rather than cultivar resistance alone, appears to be most appropriate for management of FW of watermelon. Preliminary trials in the greenhouse and field indicate that hairy vetch (_Vicia villosa_) residue amended to Fon race 2-infested soil reduced FW and increased yield but did not reduce the Fon level in soil.

Links:
<http://www.agnr.umd.edu/maes_exe/dividends/everts2.pdf>
<http://www.ipm.uiuc.edu/diseases/series900/rpd904/>
<http://www.radajamaica.com.jm/Technical/fusarium.htm>
<http://www.cals.ncsu.edu/sustainable/peet/IPM/diseases/fusarium.html>
- Mod.DH]