Fusarium oxysporum f.sp. niveum (Fusarium wilt of watermelon)

F. oxysporum f.sp. niveum causes a widespread wilt of watermelon, damping off, cortical rot and stunting of seedlings, and sudden progressive wilt of older plants. Necrotic lesions occur on the roots and browning, gum and tyloses are found in the vascular system. In mature plants, the wilt may be confined to a particular part, depending on which portion of the root system has been invaded by inoculum in the soil. Chlorosis and stunting in mature plants can occur and sometimes there is temporary recovery from wilt. Sporulation may be found on dead stems in wet weather (Holliday, 1970; Martyn, 2014).

Host-Plant Resistance

One of the best control measures against Fusarium wilt of watermelon is the use of resistant cultivars (Hopkins and Elmstrom, 1984). The cultivar Calhoun Gray is highly resistant to Fusarium wilt caused by race 1 of the pathogen in many countries and regions, including Queensland, Australia (Inch et al., 1972), Bangladesh (Mondal and Rashid, 1990), Italy (Cirulli, 1974), Montenegro (Mijuskovic and Vucinic, 1977), California, USA (Paulus et al., 1976), Florida, USA (Hopkins and Elmstrom, 1975, 1981) and South Carolina, USA (Barnes, 1972). However, after race 2 appeared in Israel and Texas, USA, Calhoun Gray became susceptible (Nepa et al., 1985; Netzer and Martyn, 1989).

In certain locations, cultivars in addition to Calhoun Gray also performed well in Fusarium-infested soil or had improved horticultural characters. In the USA, Elmstrom and Crall (1979) reported that the content of soluble solids and resistance of Dixielee watermelon to F. oxysporum f.sp. niveum was higher than in Crimson Sweet or Charleston Gray. Elmstrom and Hopkins (1981) reported that Smokylee and Summit were highly resistant, with <20% seedling wilt, and produced adequate yields even under heavy infestation. Norton et al. (1985) reported that AU-Jubilant, an inbred line from the cross Jubilee X PI271778, and AU-Producer, an inbred line from the cross Crimson Sweet X PI189225, were high-yielding and had resistance to race 1 of the pathogen, and were adapted for the southeastern part of USA. In Florida, Crall and Elmstrom (1986) developed the 'icebox' (small fruited) cultivars Minilee and Mickylee, which are resistant to the pathogen; both had a long shelf-life and appeared to be suitable for year-round production in Florida.

In China, the cultivar Calhoun was found to have resistance to Fusarium wilt (Zhang et al., 1995; Zhou and Kang, 1996a). The cultivar Jingkang 2 is resistant to Fusarium wilt and yields up to 60-75 ton/ha (Zhou, 1995). In order to quicken the procedure to find resistant breeding lines to Fusarium wilt, Huang et al. (1981) and Yu and Wang (1990) developed a simple, rapid and effective method using toxic metabolites extracted from the pathogen to screen resistant cultivars. Results showed that resistance to the toxin was correlated with resistance of the cultivars to the pathogen. A method for identifying resistance in watermelon to Fusarium wilt was recommended; this procedure combined root dip inoculation and spore culturing at the seedling stage with evaluation in the field at a later stage (Wang and Zhang, 1988; Zhang and Wang, 1991). The results of Yu et al. (1995) showed that the inheritance of resistance conformed to the additive-dominance model. The additive effect was major and the susceptibility was partially dominant.

The ability to map resistance genes coupled with marker-assisted selection is accelerating breeding watermelon cultivars resistant to race 2. Several research groups in China and the USA have identified quantitative-trait loci (QTL) located on different chromosomes and linked to resistance to race 1 or race 2. Resistance to race 1 in most modern hybrid cultivars, including Calhoun Gray, has been mapped to chromosome 1 (Yi et al., 2015; Fall et al., 2018). Branham et al. (2019) reported another race 1 resistance gene located on chromosome 9. QTLs for race 2 resistance have been mapped to chromosomes 9, 10, and 11 (Yi et al., 2015; Meru and McGregor, 2016; Branham et al., 2017).

Resistant Rootstocks

In Japan, the new bottle gourd cultivar Renshi was bred for use as a rootstock for watermelon, which prevented acute wilt of watermelon grafted on bottle gourd. It is tolerant of both very dry and wet soil. Graft compatibility with watermelon was good and the growth, quality and cropping characteristics of watermelons grafted onto Renshi were similar to those on other rootstocks (Matsuo et al., 1985).

In Korea, 19 cultivars, including Cucurbita moschata cv. Choseun, C. maxima cv. HA Sintojwa and C. pepo cv. Vegetable Spaghetti were selected as resistant to F. oxysporum f.sp. cucumerinum, niveum and melonis. A number of cultivars were selected as promising breeding lines for rootstocks, including Taeyang, Kangryeog, Strong Ilhwi and Vegetable Spaghetti. These cultivars grew at low temperatures and were resistant to Fusarium wilt (Kim et al., 1997).

In China, watermelon rootstock ChaoFeng F1 was found to be immune to Fusarium wilt, and watermelons grafted onto this rootstock matured early. Yields were 15-17.6% higher than those of watermelons grafted on a common gourd rootstock. The grafted watermelons had thinner skins and more deeply-coloured flesh (Zheng, 1995).

In Italy, D’Amore et al. (1996) reported that grafting watermelon onto rootstocks of the genera Cucurbita or Lagenaria gave protection from F. oxysporum f.sp. niveum and improved the absorption of water and nutritive elements. Yields were increased and there was an improvement in the quality of fruits.

In Bangladesh, watermelon cv. Top Yield was grafted onto nine different cucurbit rootstock seedlings: C. moschata cultivars Mammoth King, Round and Oblong; three Lagenaria leucantha (L. siceraria) cultivars; Benincasa hispida; and wild watermelon (C. maxima). Comparison was made with ungrafted watermelons. Fruit yields ranged from 13.6 kg/plant on B. hispida to 29.6 kg/plant on L. leucantha cv. Summerking, compared with 15.4 kg/plant in the ungrafted controls. Fusarium wilt was a problem only in plants on wild watermelon rootstocks (10% were affected) and in ungrafted plants (46% affected) (Mondal et al., 1994).

In the absence of watermelon cultivars resistant to Fusarium wilt caused by race 2, grafting susceptible cultivars onto interspecific hybrid squash rootstocks (C. maxima × C. moschata) or bottle gourd (L. siceraria) rootstocks protects grafted scions from Fusarium wilt (Miguel et al., 2004; Davis et al., 2008; Keinath and Hassell, 2014b). Interspecific hybrid squash and bottle gourd possess nonhost resistance to F. oxysporum f. sp. niveum races 1 and 2 (Yetıșır et al., 2003; Keinath and Hassell, 2014a). Thus, grafting is effective regardless of which race is present or predominates in a field. In Turkey, grafting ‘Crimson Tide,’ a diploid watermelon cultivar resistant to race 1, onto bottle gourd increased yields in soil infested with race 2 of F. oxysporum f.sp. niveum (Yetıșır et al., 2003). In Spain, in soil infested with unidentified races of F. oxysporum f.sp. niveum, grafting triploid watermelon onto interspecific hybrid squash ‘Shintoza’ increased yields over threefold (Miguel et al., 2004). In Mexico, grafting triploid watermelon susceptible to Fusarium wilt onto interspecific hybrid squash consistently increased total weight of fruit produced in Fusarium-infested soil (Álvarez-Hernández et al., 2015).

Biological Control

In experiments conducted in Florida, USA, soil suppressiveness of F. oxysporum f.sp. niveum occurred through more than five successive greenhouse plantings of the watermelon cultivar Florida Giant (susceptible to F. oxysporum f.sp. niveum). The authors suggested that cultivar differences were responsible for the promotion of differences in rhizosphere microflora populations associated with soil suppressiveness (Hopkins et al., 1987; Larkin et al., 1993b). Specific isolates of non-pathogenic F. oxysporum from suppressive soil were the only organisms consistently effective in reducing the disease (35-75% reduction) The mode of action of these saprophytic isolates of F. oxysporum was induced systemic resistance with biological control potential (Larkin et al., 1996).

In Taiwan, Huang et al. (1989) reported that five saprophytic isolates of fungi and 10 isolates of bacteria were obtained from watermelon roots planted in eight soils from Taiwan. F. oxysporum, F. solani and Trichoderma sp. suppressed watermelon wilt caused by F. oxysporum f.sp. niveum.

In Egypt, Michail et al. (1989) reported that Fusarium wilt of watermelon could be controlled by cross protection. Prior inoculation of plants with F. oxysporum f.sp. cucumerinum, which causes cucumber wilt disease, followed by the pathogen 5 days later resulted in no apparent wilt symptoms on watermelon. Yu and Wang (1989) also reported cross protection using a weakly virulent isolate of F. oxysporum f.sp. niveum or an isolate of F. solani to inoculate plants 5 to 15 days before challenging them with the pathogen.