High tunnels and greenhouses are protected agricultural structures that extend growing seasons in Kentucky and elsewhere in the South by providing frost protection and maintaining warmer soils. Many growers produce high value crops, like tomatoes, without crop rotation in these structures, yet sequential crops can deplete soil organic matter, increase salinity, and exacerbate soilborne pathogen issues, such as root knot nematodes (RKN). RKN (Meloidogyne spp.) are plant pathogenic roundworms with a sedentary adult female stage on roots. RKN can reduce tomato yields up to 60% in protected structures (Talavera et al., 2009).
Current RKN management strategies include soil fumigation, chemical nematicides, or resistant varieties. Fumigants are restricted-use chemicals that are most effectively applied by contracting with professional fumigation companies for a fee. Most growers are unable to use this approach due to poor access to fumigation services and incompatibility of equipment with the enclosed space of protected agricultural structures. Fumigants are also harmful to the environment and nonspecifically sterilize soils, eliminating beneficial microbes and soil animals in addition to soilborne pathogens. With nerve toxins as active ingredients, chemical nematicides effectively manage RKN populations, yet must be applied with great care as a result of potential non-target effects on applicators (Mitkowski and Abawi, 2003). Chemical nematicides may have difficulty moving through heavy soil profiles, resulting in only partial efficacy. Neither fumigation nor chemical nematicides are permissible management options for RKN in certified organic systems. Resistant tomato varieties are available to reduce the impact of southern RKN (M. incognita), however, this resistance “breaks” at high soil temperatures (Mitkowski and Abawi, 2003) and is ineffective against northern RKN (M. hapla). Grafting can be an effective strategy to grow desirable tomato varieties in RKN infested soils, however, these plants can be expensive in terms of labor and/or cost, and the rootstocks typically are not effective against northern RKN. Both southern and northern RKN are found in Kentucky soils, as well as other states within the southern region.
Long-term, economically sustainable production within protected agricultural structures is closely related to maintenance of soil quality. Measures of soil quality include soil organic matter, salinity, and pH. Organic matter stabilizes aggregates in soil, which improves texture, allows for better aeration and root system development, and promotes communities of beneficial microbes (Shepherd et al., 2002), which are components of disease-suppressive soils. High salinity and pH reduce the ability of plants to take up water and micronutrients and contribute to crusting and poor soil structure. Composted manure is one strategy to increase soil organic matter, but increasing concerns about the introduction of foodborne pathogens have led to avoidance of manure use in commercial production. Currently, growers in the Southern region lack solutions that are both effective and environmentally sustainable for simultaneous problems with RKN and soil quality in protected agricultural structures.
- To determine the efficacy of brassicaceous cover crops in reducing root knot nematode populations, augmenting high tunnel soil organic matter and salinity, and increasing tomato yields.
- To educate growers and extension agents on vegetable production and disease management in high tunnel systems.
Between proposal and project award, both of the initially-identified grower collaborators stopped producing tomatoes under the conditions set forth in the original proposal. These conditions were fairly specific: annual tomato production under high tunnels in natural soil infested with root knot nematodes. One producer converted both high tunnels in tomato production to a bagged, soilless system as a direct result of the root knot nematode infestation. The other producer, phasing into retirement, chose to utilize his high tunnel in other, less labor-intensive ways. During spring and summer 2016, the primary investigator sought alternate sites to accomplish the project, proposed to commence in October 2016.
To identify other tomato producers with root knot nematode pressure in high tunnels, the University of Kentucky Plant Disease Diagnostic Lab Database was searched. Extension agents and growers were contacted to determine the suitability of sites. Several potential sites were deemed non-commercial or too small to accommodate the proposed replicated research trial. Several other sites were no longer producing tomatoes in soil, converting to the bagged system. Another site converted from tomato to hanging basket ornamental production as a result of RKN-infested soils. The site that held highest promise was visited in July 2016, only to determine that in addition to root knot nematode, this grower had simultaneous, significant disease pressure from Southern blight. Southern blight is caused by a fungal pathogen that persists in soil for years and has an extremely broad host range; unfortunately, this pathogen can increase on the cover crops intended to be tested for nematode management. Simultaneously occurring diseases would confound data collection, in addition to potentially jeopardizing the producer’s future crops in the structure by increasing Southern blight. For these reasons, this site was also considered unacceptable. Sites suggested by the primary investigator’s predecessor were also determined to be unable to accommodate the project.
Due to the inability to find alternate sites after several months’ search, this project was terminated prior to initiating the trials. Perhaps the most striking outcome of this short experience was that numerous Kentucky growers completely abandoned tomato production in infested high tunnel soils. This reiterates the severity of the problem of root knot nematode-infested soils, and emphasizes the need for sustainable solutions for nematode management in high tunnel systems.