Fusarium Wilt Of Watermelon Research Articles

Allelopathic effects of root exudates from watermelon and rice plants on Fusarium oxysporum f.sp. niveum

Root exudates have a key role in communication between plants and microbes in the rhizosphere. Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (Fusarium oxysporum), drastically reduces watermelon yields in continuous cultivation systems, but it can be significantly alleviated using watermelon/aerobic rice intercropping system as shown by the research carried out in this laboratory. It is important to evaluate the interaction between root exudates from the two crops and the pathogen and thus to clarify the mechanism of disease suppressiveness in the intercropping system. The effects of phenolic acids, sugars and free amino acids in root exudates from watermelon (REW) and rice (RER) on the growth of Fusarium oxysporum were studied. The results obtained are listed as follows: (1) REW significantly increased spore germination and sporulation, whereas RER had inhibitory effects on those two parameters. (2) HPLC analysis showed that salicylic acid, p-hydroxybenzoic acid and phthalic acid were identified in exudates from both plants, but p-coumaric acid was only detected in rice and ferulic acid only in watermelon. Moreover, of the total rice exudates a high proportion (37.9 %) of p-coumaric acid was detected and the total amount of phenolic acids was 1.4-fold as high as that in watermelon. (3) Considerable differences in the components and contents of both sugars and amino acids were found between REW and RER exudates. (4) Exogenously applied alanine (Ala) increased spore germination and sporulation. In contrast, addition of exogenous p-coumaric acid reduced spore germination and sporulation, relative to controls. It was concluded that the rice root exudates had anti-fungal properties while that from watermelon promoted pathogen growth. This discovery provided a scientific basis for practicing watermelon/aerobic rice intercropping to control Fusarium wilt in watermelon.

POTENTIAL IMPACT OF A NEW HIGHLY VIRULENT RACE OF FUSARIUM OXYSPORUM F. SP. NIVEUM IN WATERMELON IN THE USA

Fusarium wilt of watermelon was first reported in the United States (U.S.) in 1894. Although variation in virulence within the pathogen population, Fusarium oxysporum f. sp. niveum, had been observed, differentiation of isolates into races did not occur for 70 years. Currently, three races (0, 1, and 2) of F. oxysporum f. sp. niveum have been described. Races 0 and 1 were first described in 1963 and race 2 was described in 1973. Although race 1 is the predominant race, race 2 appears to be spreading in the U.S. and its presence has been confirmed in eight states. Losses from Fusarium wilt were managed for many years through the use of cultivars that were resistant to race 1. However, the prevalence and severity of Fusarium wilt in watermelon has been increasing in the eastern U.S. in the past decade. This increase has coincided with a large increase in production of triploid watermelon cultivars. Most triploid cultivars evaluated in 2002 and 2005 in a field with a mixture of races 1 and 2 were rated susceptible to Fusarium wilt. In 2009 cultivars which were reported to be resistant to F. oxysporum f. sp. niveum race 1 were evaluated and many also exhibited severe wilt. During surveys of the watermelon production region in Maryland and Delaware from 2000 to 2007, some isolates of F. oxysporum f. sp. niveum were found that appeared to be more virulent than any of the previously described races. Two isolates of F. oxysporum f. sp. niveum collected in Maryland in 2001 along with reference isolates of races 0, 1, and 2 were compared for virulence, host range and vegetative compatibility. Race identification was made on eight differential watermelon genotypes. Both Maryland isolates were highly virulent, causing 65-100% wilt on all differentials, two of which were PI-296341-FR (FL) and PI-296341-FR (OK) considered highly resistant to race 2. Both isolates were non-pathogenic on muskmelon, cucumber, pumpkin and squash. Two additional isolates collected in Maryland in 2007 also were tested. These studies demonstrated that the four Maryland isolates were more virulent than race 2 and are proposed as a new race 3. Although most fields surveyed had a predominant population of race 1, the presence of F. oxysporum f. sp. niveum race 2 and the discovery of race 3 highlight the need for the development of new resistant cultivars.

Effect of exogenously added ferulic acid on in vitro Fusarium oxysporum f. sp. niveum

Abstract Allelochemicals from root exudates or decaying residues of watermelon plant may be related to watermelon fusarium wilt. The aim of this work was to study the effect of an artificially applied allelochemical, ferulic acid, on in vitro Fusarium oxysporum f. sp. niveum (FON), a causal pathogen of plant wilting in the laboratory setting. The results showed that ferulic acid inhibited growth at high concentrations. The biomass was reduced by 71.6% and the conidial germinate rate was decreased by 100%, while mycotoxin production by FON was increased by 227.7% at the highest concentration (1600 mg L−1). Activities of hydrolytic enzymes related to pathogenicity were also affected. It is concluded that ferulic acid at commonly found concentrations inside plants suppressed the growth of pathogen F. oxysporum f. sp. niveum.

Cover Crops of Hybrid Common Vetch Reduce Fusarium Wilt of Seedless Watermelon in the Eastern United States

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum races 0, 1, 2, or 3, is found throughout the eastern United States. Most seedless (triploid) watermelon cultivars are susceptible to all races. In previous studies in Maryland, winter cover crops of hairy vetch (Vicia villosa) reduced Fusarium wilt. The objectives of this study were to compare ‘Cahaba White’ hybrid common vetch (V. sativa × V. cordata) to hairy vetch for suppression of Fusarium wilt in South Carolina and Maryland. Cover crops of the two vetches and rye (the control) were seeded in fall 2006 and 2007 in fields naturally infested with a mixture of races 1 and 2 in South Carolina and Maryland. In the spring after incorporating the cover crop biomass, seedless watermelons cv. Sugar Heart (susceptible) or Revolution (moderately resistant to race 1) were transplanted into subplots within each cover crop whole plot. ‘Cahaba White’ vetch was as effective as hairy vetch in reducing incidence of Fusarium wilt. Both vetch varieties reduced wilt incidence by approximately the same degree with the susceptible as with the moderately resistant cultivar. In general, the moderately resistant cultivar yielded more fruit (by weight) than the susceptible cultivar. Accepted for publication 23 August 2010. Published 14 September 2010.

A simplified technique for grafting watermelon onto resistant cucurbit rootstocks for control of Fusarium wilt of watermelon in Nghe An Province, Vietnam

Resistant cucurbit rootstocks provided an effective control measure for Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum in Nghe An province, Vietnam. The hybrid cultivar, Bulrojangsaeng (Lagenaria siceraria) was the most suitable rootstock on all criteria but the seed is expensive. Therefore, the local Bau trang cultivar (L. siceraria) was adopted by farmers as the preferred rootstock on the basis of the low-cost seed. The grafting process is described and illustrated.

First report of Fusarium wilt of watermelon in Vietnam

Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum caused seedling losses in nurseries, as well as severe losses in many crops in Nghe An province, Vietnam, in 2008. Isolates of the fungus were shown to be pathogenic. All 20 cultivars grown in the province in 2008 were susceptible. This is the first formal report of this disease in Vietnam.

Suppression of Fusarium wilt of watermelon by a bio-organic fertilizer containing combinations of antagonistic microorganisms

Fusarium wilt of watermelon commonly occurs in locations where the crop has been grown for many seasons. Its occurrence results in a severely decreased watermelon crop. The goal of this study was to assess the capability of a new product (bio-organic fertilizer) to control the wilt in Fusarium-infested soil. Pot experiments were conducted under growth chamber and greenhouse conditions. The results showed that the fertilizer controlled the wilt disease. Compared with control pots, the incidence rates of Fusarium wilt at 27 and 63 days following treatment of the plants with the bio-organic fertilizer at a rate of 0.5% (organic fertilizer + antagonistic microorganisms, including 3 × 109 CFU g−1Paenibacillus polymyxa and 5 × 107 CFU g−1Trichoderma harzianum) were reduced by 84.9 and 75.0%, respectively, in both the growth chamber and greenhouse settings. The activities of antioxidases (catalase, superoxide dismutase and peroxidase) in watermelon leaves increased by 38.9, 150 and 250%, respectively. In the roots, stems and leaves, the activity of β-1,3-glucanase (pathogenesis-related proteins) increased by 80, 1140 and 100% and that of chitinase increased by 240, 80, and 20%, respectively, while the contents of malondialdehyde fell by 56.8, 42.1 and 45.9%, respectively. These results indicate that this new fertilizer formula is capable of protecting watermelon from Fusarium oxysporum f.sp. niveum. The elevated levels of defense-related enzymes are consistent with the induction and enhancement of systemic acquired resistance of plant.

Effect of fungal fusaric acid on the root and leaf physiology of watermelon (Citrullus lanatus) seedlings

The impact of fusaric acid (FA) phytotoxin on the physiology of root leaf cells in watermelon seedlings was evaluated. Results revealed that the cell membrane potential treated with FA in 12 h was decreased by 61.9–81.8% compared with untreated controls. FA markedly accelerated the lipid peroxidase activity of watermelon leaves. Malondiadehyde in leaves treated with different concentrations of FA in 24 h was 5.2–11.0 fold as much as control. Phenylalanine ammonia-lyase activity (PAL) in leaves treated with FA was first increased and then decreased. The highest PAL activity was obtained after 6–12 h, which was 7.2–10.5 fold as much as control. Activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) were first increased and then decreased when the seedlings were treated with FA. The highest SOD and POD activities were observed after 12 h and then both declined. The β-1,3-glucanase and chitinase activities in leaves treated with FA were first quickly increased and then declined, the highest activities were found after 12 and 3 h. We concluded that FA strongly inhibited root and leaf cells functions physiologically responsible for fusarium wilt of watermelon.

Effect of Incorporation of Brassica spp. Residues on Population Densities of Soilborne Microorganisms and on Damping-off and Fusarium Wilt of Watermelon.

Incorporating Brassica spp. residue to reduce populations of soilborne fungi and manage damping-off and Fusarium wilt of watermelon (Citrullus lanatus var. lanatus) was studied in two field experiments. Treatments included incorporating flowering Brassica napus cv. Dwarf Essex canola or B. juncea cv. Cutlass mustard and laying black polyethylene mulch at incorporation or 1 month after incorporation, methyl bromide, and a nontreated control. In both years, glucosinolates were identified and quantified in the shoots and roots of the flowering plants. In both years, the total concentration of glucosinolates incorporated per square meter was significantly higher for B. juncea than for B. napus. Isothiocyanates were inconsistently detected in the amended soils and none were detected more than 12 days postincorporation. After incorporation in 2004 and 2005, amended plots had higher populations of Fusarium oxysporum and Pythium spp. than the methyl bromide treatment, and in some treatments, populations were higher than in the control. Fluorescent Pseudomonas spp. were not suppressed in amended soils, and their populations were significantly higher in some amended treatments than those in methyl bromide-treated soils or nontreated control soils. Incidence of damping-off and severity of Fusarium wilt on seedless watermelon cv. Tri-X 313, which is susceptible to Fusarium wilt, were not consistently lower in brassica-amended soils or methyl bromide-treated plots than in nontreated control plots. Therefore, under spring conditions and methods used in this study, neither biofumigation nor methyl bromide fumigation in coastal South Carolina was an effective disease management tool for two soilborne pathogens of watermelon.

Cinnamic Acid Inhibits Growth but Stimulates Production of Pathogenesis Factors by in Vitro Cultures of Fusarium oxysporum f.sp. niveum

Long-term monoculture of watermelon leads to frequent occurrence of watermelon fusarium wilt caused by Fusarium oxysporum f.sp. niveum (FON). Some allelochemicals contained in watermelon root exudates and decaying residues are possibly responsible for promoting the wilt disease. The purpose of this study was to evaluate the allelopathic effect of artificially applied cinnamic acid on FON. Results demonstrated that hyphal growth of FON was strongly inhibited by cinnamic acid. At the highest concentration of cinnamic acid, the biomass in liquid culture was decreased by 63.3%, while colony diameter, conidial germination on plates, and conidial production in liquid culture were completely inhibited. However, mycotoxin production and activity of phytopathogenic enzymes were greatly stimulated. Mycotoxin yield, pectinase activity, proteinase activity, cellulase activity, and amylase activity were increased by 490, 590, 760, 2006, and 27.0%, respectively. It was concluded that cinnamic acid dramatically stimulated mycotoxin production and activities of hydrolytic enzymes by FON but inhibited growth and germination of FON. The findings presented here indicate that cinnamic acid is involved in promoting watermelon fusarium wilt.

Reduction of Fusarium wilt in watermelon by Pseudomonas chlororaphis PCL1391 and P. fluorescens WCS365

Fusarium wilt of watermelon (Citrullus lanatus) caused by Fusarium oxysporum f. sp. niveum is a devastatine soil-borne disease that causes extensive losses throughout the world. Two known Pseudomonas biocontrol strains were used separately and in combination to assess their antagonistic effectiveness against F. oxysporum f. sp. niveum in pot experiments. P. chlororaphis PCL1391 signifi cantly reduced disease severity. P. fl uorescens WCS365 was less effective in disease suppression, while a combination of the two bacteria had intermediate effects. The biological control of Fusarium wilt with P. chlororaphis offers a potentially useful tool in an integrated pest management program to control Fusarium wilt of watermelon.

Intercropping with aerobic rice suppressed Fusarium wilt in watermelon

Watermelon is susceptible to Fusarium wilt in successively mono-cropped soil. Pot experiments were carried out to investigate the effect of intercropping with aerobic rice on Fusarium wilt in watermelon. The tested soil was classified as a loam soil, previously planted with watermelon and collected from Hexian county, Anhui province, China. The results obtained are listed as follows: (1) 66.7% of watermelon plants were infected with wilt disease and 44.4% died on 40 days after transplanting in mono-cropped soil, but plants were much less susceptible to infection when intercropped with rice; (2) the density of Fusarium oxysporum f. sp. niveum decreased by 91% in soil from the intercropped watermelon rhizosphere when compared with that from the mono-crop 40 days after transplanting; (3) densities of bacteria and actinomycetes increased, but fungal density decreased in rhizosphere soil from the intercrops in comparison with the mono-crop control; (4) compared to the control, the germinated Fusarium spores were decreased by 41.0% in the treatment with addition of 1.5 ml rice root exudates. Adding 20 ml of root exudates decreased Fusarium spore production by 76.4%; and (5) the activities of defense enzymes in the leaves and roots of watermelons in the intercropped system were significantly lower than those in the mono-cropped system. It is suggested that intercropping with aerobic rice alleviated Fusarium wilt in watermelon, by restraining the spore production of Fusarium and by changing the microbial communities in rhizosphere soil through the production of rice root exudates.

Effects of Host Resistance and Inoculum Density on the Suppression of Fusarium Wilt of Watermelon Induced by Hairy Vetch.

Hairy vetch (Vicia villosa) used as a soil amendment is a newly described potential management tool for the suppression of Fusarium wilt of watermelon (Citrullus lanatus). However, the effect of inoculum density and the level of resistance in the host on the level of suppression are not understood. In this study, hairy vetch-induced wilt suppression was evaluated in the greenhouse on 12 watermelon cultivars with different levels of wilt resistance and in 16 naturally infested soil samples collected from commercial watermelon fields. Wilt suppression occurred in all but two cultivars and with the trend that suppression increased as the level of resistance in cultivars increased. Fusarium wilt suppression was 22, 53, and 63% in hairy vetch-amended soil compared with nonamended soil on cultivars ranked as susceptible, moderately resistant, and highly resistant, respectively. Suppression also occurred in nine of the soils that contained populations of Fusarium oxysporum f. sp. niveum below 1,100 CFU/g of soil. However, at this level or higher, significant wilt suppression was not observed. The magnitude of disease suppression decreased with the increase of inoculum in the soils. The induced wilt suppression appeared to be correlated with an increase in bacterial populations in soil. Hairy vetch-induced suppression to Fusarium wilt in watermelon is dependent on the resistance level of cultivars and is overcome by high inoculum level of F. oxysporum f. sp. niveum in soil.

Suppression of Fusarium Wilt of Watermelon Enhanced by Hairy Vetch Green Manure and Partial Cultivar Resistance

Hairy vetch (Vicia villosa Roth) green manure is a newly-described potential management tool for Fusarium wilt of watermelon, but control is insufficient when watermelon, especially triploid watermelon, is grown in severely infested soils. A field experiment in a split-split-plot design was conducted over two years to evaluate efficacy of hairy vetch green manure alone and in combination with a moderately wilt-resistant (MR) triploid watermelon cultivar for wilt suppression compared with preplant soil fumigants. Either the soil-incorporated hairy vetch winter cover crop or the MR cultivar was effective in reducing wilt incidence, promoting plant vine growth, and increasing fruit yield. However, neither approach alone resulted in disease reductions sufficient to obtain an acceptable level of marketable fruit yield. An additive effect was observed when both treatments were combined and was greater than that obtained with the fumigants methyl bromide or metam sodium. Stem colonization by Fusarium oxysporum f. sp. niveum was lower following hairy vetch green manure than in fallow treatments, and was lowest in the MR cultivar grown in green-manured plots. The combined use of hairy vetch green manure and a MR cultivar can enhance suppression of Fusarium wilt in triploid watermelon. Accepted for publication 25 February 2006. Published 5 April 2006.

Suppression of Fusarium Wilt of Watermelon by Soil Amendment with Hairy Vetch.

Hairy vetch (Vicia villosa Roth) as a soil amendment was evaluated for suppression of Fusarium wilt of watermelon and soil populations of Fusarium oxysporum f. sp. niveum in greenhouse, microplot, and field studies. When mixed at 1 or 5% (wt/wt) in a loamy sand soil that was artificially or naturally infested with race 2 of F. oxysporum f. sp. niveum, pulverized dry hairy vetch, crab shell, and urea provided the best suppression (53 to 87% reduction) of Fusarium wilt on watermelon seedlings among 13 plant and animal residues screened. Soil amended with hairy vetch at 0.25 or 0.5% (wt/wt) in microplots resulted in 54 to 69% decreased wilt incidence and 100 to 220% increase of watermelon plant biomass. Hairy vetch winter cover crop incorporated into field plots under black plastic provided 42 to 48% reduction of wilt incidence, 64 to 100% increase of plant biomass, and a 34 to 68% increase in weight of fruit, comparable to improvements achieved by the soil fumigants methyl bromide or 1,3-dichloropropene plus 35% chloropicrin. Soil amendment with hairy vetch also increased the sugar content of watermelon fruit 10 to 15%. Significant reductions in the populations of F. oxysporum f. sp. niveum were not observed in hairy vetch-amended soil in microplots and field plots, but were observed in greenhouse pot soil amended with 5% (wt/wt) hairy vetch, which was attributed primarily to increased levels of fungicidal ammonia produced during decomposition. Incorporating hairy vetch into mulched soil can be an alternative or supplement to cultivar resistance and crop rotation for management of Fusarium wilt of watermelon.

Races and Inoculum Density of Fusarium oxysporum f. sp. niveum in Commercial Watermelon Fields in Maryland and Delaware.

A survey was conducted to determine races and inoculum density of Fusarium oxysporum f. sp. niveum, the causal agent of Fusarium wilt of watermelon in Maryland and Delaware. Virulence on six differential cultivars was tested for each of 63 isolates of F. oxysporum f. sp. niveum, obtained from 25 commercial watermelon fields. Thirteen isolates (21%) were identified as race 0, 36 isolates (57%) as race 1, and 14 isolates (22%) as race 2. Races 0 and 1 were present in 12 (48%) and 10 (40%) of the fields, respectively. The highly aggressive race 2 was identified from five fields in two counties in Maryland and from one field in Delaware, representing 24% of the fields. Race 2 was copresent with one or two other races. Race 2 (19 isolates) predominated among the 25 isolates obtained from a research field in Maryland. Nineteen commercial fields had inoculum densities of F. oxysporum f. sp. niveum ranging from 100 to 1,200 CFU/g of soil at harvest. Within this range of inoculum densities, >20% incidence of wilt was observed when the susceptible watermelon cv. Sugar Baby was planted in samples of soil collected from these fields. The relationship (P < 0.0001) between inoculum density of F. oxysporum f. sp. niveum (X) and incidence of Fusarium wilt (Y) on Sugar Baby was best described using the monomolecular equation, Y = 1 - exp[-0.0013 (X + 166)]. The ratio of pathogenic to total population of F. oxysporum in the fields linearly increased with increasing inoculum density of F. oxysporum f. sp. niveum (R 2 = 0.4; P < 0.0009).

국내 토양병해 발생현황과 종합 관리방안

최근 연작장해의 주원인으로 지적되는 토양병해의 발생은 매우 심각한 양상을 띄고 있다. 그 예로 배추 무사마귀병, 마늘 흑색썩음균핵병, 감자의 풋마름병, 고추 역병, 토마토 시들음병 , 수박의 CGMMV 등을 들 수 있으며 이 모두 시급히 해결하지 안되는 현실문제다. 토양병 발생이 이렇게 심각해진 것은 특정작물의 주산단지화 및 주년 생산체계, 시설원예의 보편화에 의한 획일적인 작부체계 내지는 동일작물 연작에 의해 토양환경이 급속히 악화된데 그 근본 원인이 있으며 이외에 수량 및 품질위주의 품종 육성에 의해 작물의 병 저항성을 소홀히 한 점이나 화학비료 위주의 경종방법 등도 토양병 발생을 부추기는 한 원인으로 지적되고 있다. 토양병 발생에 대한대책으로 그 원인을 찾아 제거하는 방법이 가장 효율적이라고 생각되는바 장기적인 대책으로 지역의 현실여건 및 경제성을 고려한 합리적 작부체계의 개발·운용, 작물별로 피해가 심한 다수 병해를 대상으로 한 복합저항성 품종의 육성 ·보급, 농토배양 및 토양개량을 통한 악화된 토양환경의 개선을 들 수 있고 단기적인 대책으로는 간편한 토양소독 기술의 개발·보급, 신속·정확한 토양병 예찰기술의 정립이 긴급하다. 이를 위하여 토양병 전문연구자의 육성 및 현장문제 해결을 위한 공동연구의 활성화 등에 국가적인 차원에서의 적극적인 지원이 요청되고 있다. Incidence of soilborne diseases, as a major cause of failure of continuous monocropping becomes severe in recent years. For examples, recent epidemics of club root of chinese cabbage, white rot of garlic, bacterial wilt of potato, pepper phytophthora blight, tomato fusarium wilt and CGMMV of watermelon are the diseases that require urgent control measures. Reasons for the severe incidence of soilborne diseases are the simplified cropping system or continuous monocropping associated with allocation of major production areas of certain crop and year-round cultivation system that results in rapid degradation of soil environment. Neglect of breeding for disease resistance relative to giving much emphasis on high yield and good quality, and cultural methods putting first on the use of chemical fertilizers are thought to be the reason. Counter-measures against soilborne disease epidemics would become most effective when the remedies are seeded for individual causes. As long-term strategies, development of rational cropping system which fits local cropping and economic condition, development and supply of cultivars resistant to multiple diseases, and improvement of soil environment by soil conditioning are suggested. In short-term strategies, simple and economical soil-disinfestation technology, and quick and accurate forecasting methods for soilborne diseases are urgent matter far development. for these, extensive supports are required in governmental level for rearing soilborne disease specialists and activation of collaborating researches to solve encountering problems of soilborne diseases.

Novel Use of a Pyrenomycetous Mycoparasite for Management of Fusarium Wilt of Watermelon

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is a destructive disease that limits watermelon production in many areas of the world. The discovery of several pyrenomycetous ascomycetes occurring naturally in association with different formae speciales of F. oxysporum identified these fungi as potential biological control organisms for watermelon wilt. One such mycoparasitic isolate, identified as Sphaerodes retispora var. retispora, was chosen for biological control and ecological trials in the greenhouse. Four different methods to inoculate the mycoparasite were evaluated, three of which utilized the parasite encapsulated into sodium alginate pellets. The other method employed root-dipping plants with mycoparasite ascospore suspensions. Ecological factors also were investigated, including the ability of S. retispora var. retispora to colonize watermelon roots, and its ability to survive in soil over time and reduce propagules of F. oxysporum f. sp. niveum. In the biological control studies, the use of the mycoparasite significantly reduced plant mortality and increased dry weights of watermelon plants after being challenged with F. oxysporum f. sp. niveum, compared with pathogen-inoculated controls. It appears that the incorporation of the parasite into alginate pellets in the planting mix at seeding may be the most practical method for future field evaluations of transplant-grown vegetable crops. In the ecological studies, the mycoparasite was recovered from infested soil after 9 months, but was only isolated from watermelon roots when applied in the presence of F. oxysporum. S. retispora var. retispora had no effect on F. oxysporum f. sp. niveum propagules after being applied to soils in the greenhouse.

First Report of the Occurrence of Fusarium oxysporum f. sp. niveum Race 2 in Commercial Watermelon Production Areas of Maryland and Delaware.

Resistance to race 1 of Fusarium oxysporum f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans) is the most widely used tool for management of Fusarium wilt of watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai). However, this resistance is ineffective against the more aggressive F. oxysporum f. sp. niveum race 2. Race 2 was first identified in Israel in 1973 (2) and was subsequently reported in Texas (1981), Oklahoma (1988), and Florida (1989) (1). From July through September 2000 in Maryland and Delaware, 123 isolates of F. oxysporum f. sp. niveum were collected from wilted plants or plants with vascular discoloration from commercial production fields with moderate to severe wilt severity. Race determination was performed in the greenhouse on five race-differential cultivars. Differential cvs. Sugar Baby, Crimson Sweet, Charleston Gray, Allsweet, and Dixielee range from susceptible to highly resistant (in the order listed) to race 1, but all are susceptible to race 2. Seeds were planted in pots containing pasteurized vermiculite and peat moss (2:3 mixture) growth medium infested with 1 × 105 conidia per g of growth medium. Conidia were harvested from 5- to 6-day-old cultures of F. oxysporum f. sp. niveum grown in a liquid mineral salts medium (2). Control pots were treated with water or diluted liquid medium. Pots were maintained in a greenhouse at 19 to 28°C. Three replicate pots containing seven plants each were used for each isolate-cultivar combination. Race 0 obtained from B. D. Bruton (USDA-ARS, Lane, OK), and races 1 and 2 obtained from the American Type Culture Collection (Manassas, VA) were included in the tests for comparative purposes. Eleven isolates from Maryland and three isolates from Delaware consistently caused 60 to 100% wilt or mortality of all cultivars, with a mean of more than 75% wilt. Percent wilt of cvs. Sugar Baby and Dixielee to races 0, 1, and 2, and the 14 isolates from Maryland and Delaware was 78, 76, 100, and 95 to 100% and 0, 14, 59, and 63 to 93%, respectively. All control plants remained healthy. In each of the three replicated tests, these 14 isolates caused an equal or greater incidence of wilt as the reference race 2 isolate, and cvs. Dixielee and Allsweet were susceptible to these isolates. Therefore, these isolates were identified as F. oxysporum f. sp. niveum race 2. This is the first report of F. oxsporum f. sp. niveum race 2 occurring in Maryland and Delaware.

Fusarium wilt of watermelon in Cyprus and its management with soil solarization combined with fumigation or ammonium fertilizers

Fusarium wilt is a limiting factor to watermelon production in Cyprus. In previous studies, soil solarization applied in 1-m-wide strips provided only partial disease control. In order to improve the effectiveness of solarization, three field experiments were carried out in heavily infested soils, using various solarization techniques, alone or in combination with methyl bromide fumigation or soil amendments with ammonium-based fertilizers. Increasing the width of the solarization strip from 1 to 2 m had no significant influence on its effectiveness. Solarization with a double rather than single polyethylene sheet, in either 1- or 2-m-wide strips, improved slightly the level of wilt control but had no significant effect on yield. Combined solarization–methyl bromide treatments, however, improved significantly the level of wilt control and increased marketable yield by up to 100% over that obtained with either method alone. The low rate of methyl bromide (50gm−2) was as effective as the normal rate used by growers (100 g m−). Soil amendment with ammonium-based fertilizers improved even more the effectiveness of solarization and increased yield by up to 200% over that obtained with solarization alone. Best results were obtained with ammonium sulphate or monoammonium phosphate at a rate equivalent to 180 kg N ha−1.