Macro- and Microelement Fertilizers Influence the Severity of Fusarium Crown and Root Rot of Tomato in a Soilless Production System

Host nutritional variables were evaluated for their effects on the severity of crown and root rot of tomato caused by Fusarium oxysporum f.sp. radicis-lycopersici. Tomato (Lycopersicon esculentum Mill.) seedlings (cv. Bonnie Best) were grown in a pathogeninfested, soilless rockwool system in the greenhouse and were fertilized with a nutrient solution that was amended with macroand microelements at various rates. Disease was evaluated after 2 weeks using an index of 0 to 4, and plant fresh weight was measured. Regression analysis indicated that disease severity was significantly increased by ammonium-nitrogen [NH4Cl, (NH4)6Mo7O24, and (NH4)2SO4], NaH2PO4·H2O, Fe-EDDHA, MnSO4, MoO3, and ZnSO4·7H2O. Disease severity was reduced by nitrate-nitrogen [Ca(NO3)2·4H2O] and CuSO4·H2O. Low rates of NH4NO3 (39 to 79 mg·L N) reduced disease, but rates above 100 mg·L N increased it. Disease was not affected by MgSO4·7H2O. In all cases, plant growth was inversely related to disease severity. Mineral fertilizers had no effect on nutrient solution pH. This information sheds new light on environmental factors that influence plant–pathogen interactions, and may be applied to develop a management strategy for Fusarium crown and root rot based on host nutrition. Jarvis and Thorpe, 1981), but is technically impractical in hydroponic production systems. Alternative control measures are needed, ideally with the aim of developing an integrated disease management strategy. The nutritional status of a plant has a major impact on disease susceptibility, and this has been exploited for suppressing a variety of diseases (Engelhard, 1989). Notable examples are Fusarium wilts caused by F. oxysporum formae speciali, with reports dating to the 1920s that describe the beneficial effect of lime amendments (Jones et al., 1989). Since then, the effects of most major and minor nutrients on wilt diseases have been studied. Jones and coworkers (Jones et al., 1989) applied this information to develop an effective fertilizer-based management strategy for Fusarium wilt of tomato that is used in commercial production systems in the United States. Similar approaches have been successful for control of Fusarium wilts of other vegetable and ornamental crops (Elmer, 1992; Jones et al., 1989; Schneider, 1985). In contrast, little is currently known about the influence of plant nutrition on crown and root rot of tomato. Previous studies tested single concentrations of nitrogenous fertilizers and sodium chloride (Jarvis and Thorpe, 1980; Woltz et al., 1992), and did not examine the influence of other potentially critical mineral nutrients, particularly microelements. The objective of our study was to compare the effects of several macroand microelements (various ammonium-N sources, nitrate-N, sodium phosphate, iron, molybdenum, and copper-, magnesium-, manganese-, and zincsulfates) on disease severity and growth of tomato seedlings. Mineral nutrients were tested across a range of concentrations to provide more information that can be applied to develop a fertilizer-based management strategy and for integrating mineral fertilizers with other control approaches (e.g., biocontrol) that may be sensitive to mineral levels. Materials and Methods The influence of various mineral nutrients on Fusarium crown and root rot was evaluated in a noncirculating hydroponics system. Pregerminated [2–3 d on 0.85% water agar (Oxoid, Hampshire, England) at 24 °C; 2–4 mm-long primary root] tomato seeds cv. Bonnie Best were planted on rockwool cubes (3.5 cm diameter × 4 cm deep; one seed per cube; Grodania A/S, Hedehusene, Denmark) in square plastic trays (5.5 cm deep). The rockwool was saturated with 800 mL of dilute (1/4 strength) Knop nutrient solution (Ziegler, 1983) containing (mg·L): Ca(NO3)2·4H2O (250); KH2PO4, KCl, and MgSO4·7H2O (62.5 each); and Fe-EDDHA [ethylenediamine-di(ohydroxyphenyl-acetic acid); Sequestrene 138 Fe, Novartis AG, Basel, Switzerland] (5). Autoclave-sterilized stock solutions of the various minerals tested [Ca(NO3)2·4H2O, CuSO4·5H2O, MgSO4·7H2O, MnSO4, MoO3, NaH4PO4·H2O, NH4Cl, (NH4)6Mo7O24·4H2O, NH4NO3, (NH4)2SO4, ZnSO4·7H2O] were added to the nutrient solution just before use to Crown and root rot of tomato (also referred to as foot and root rot) caused by Fusarium oxysporum Schlechtend.:Fr. f.sp. radicislycopersici Jarvis & Shoemaker was first described in Japan in the late 1970s, and has since become an economically important problem in greenhouse tomato production throughout Europe, North America, Japan, and Israel (Jarvis, 1988). It is considered the most destructive tomato disease caused by a nonzoosporic pathogen in soilless hydroponics production systems, where reductions in marketable yield can exceed 60% (Mihuta-Grimm et al., 1990; Stanghellini and Rasmussen, 1994). Tomato plants can be infected at any time, but losses are especially severe when infection occurs at the seedling stage. Symptoms include dark brown necrotic lesions that form on the roots and crown region and may extend up the hypocotyl, stem, and foliage; localized discoloration of the vascular tissues; and subsequent leaf chlorosis, wilting, and death. The pathogen is spread via infected transplants, which often carry latent infections. It can then grow to some extent through soil to infect adjacent plants (Hartman and Fletcher, 1991; Louter and Edgington, 1990). The threat to tomato production is further increased by infection from airborne conidia that are produced in stem lesions and are dispersed by wind and fungus gnats [Bradysia sp. (Diptera: Sciaridae)] (Gillespie and Menzies, 1993; Rowe and Farley, 1981). Control of Fusarium crown and root rot on tomato has been difficult and will probably require multiple approaches. Only a few resistant cultivars are available (Erb and Rowe, 1992; Heremans, 1996; Sugahara and Sakurai, 1991). Fungicides, including benomyl, captafol, imazalil, thiram, and prochloraz-Mn, provide inconsistent control, leave problematic residues in edible tissues, and are often phytotoxic even when applied at recommended rates, especially on seedlings (Hartman and Fletcher, 1991; Jarvis, 1988, 1992; MihutaGrimm et al., 1990). Biological control utilizing fungal (i.e., Trichoderma harzianum and nonpathogenic Fusarium oxysporum and F. solani) and bacterial agents (i.e., Bacillus subtilis and Pseudomonas sp.) typically provides only a moderate level of disease suppression in the greenhouse and field (Bochow et al., 1996; Duffy and Défago, 1997; Hartman and Fletcher, 1991; Louter and Edgington, 1990; M’Piga et al., 1997; Sivan et al., 1987). Allelopathy from lettuce (Lactuca sativa L.) and dandelion (Taraxacum Weber sect. ruderalia sp. Kirschner) residues incorporated into soil provides limited control (Jarvis, 1992; Received for publication 3 Aug. 1998. Accepted for publication 21 Oct. 1998. We thank Barbara Pärli and Ingeborg Kump for technical assistance, Jacques Fuchs and Marcello Zala for valuable advice in developing the disease assay, and Emmannuel Frossard, ETH Zürich, and David Weller, USDA– ARS Pullman, Wash., for comments on the manuscript. 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