W Elmer

The Connecticut Agricultural Experiment Station, New Haven, Connecticut, USA

Background and objectives
Asparagus (Asparagus officinalis L.) is susceptible to a crown and root rot disease that is caused by Fusarium oxysporum and F. ;proliferatum. This disease is a major limitation on production of asparagus and occurs wherever asparagus is grown. In the USA, it became severe during the 1940s and 1950s. Prior to the 1940s, asparagus fields in the USA were dressed with NaCl to suppress weeds since asparagus is very tolerant of salt. This practice was phased out after the advent of synthetic herbicides in 1941. We found that applications of NaCl (560-1120 ;kg/ha) on asparagus fields exhibiting disease symptoms reduced the disease severity and increased yields [1]. No fungicidal activity toward the pathogens was observed with these rates of NaCl. Field tests found that NaCl-treated asparagus plants had greater numbers of Mn-reducing pseudomonads colonizing the roots and had more Mn in the roots than roots of nontreated plants [2]. Since Mn is associated with disease resistance in plants, it was hypothesized that the NaCl may function in disease resistance by promoting these microorganisms in the rhizosphere. It is possible that the increase in the Mn-reducing pseudomonads was the result of changes in root content and root exudates. The objective of this report is to examine the effect of NaCl on the composition of roots and root exudates from asparagus.

Materials and methods
Healthy 2-month-old asparagus ('Mary Washington') transplants were grown in a 1-l pots filled with a pasteurized soil-sand mix infested with and without the Fusarium pathogens. Granular applications of 1 g of NaCl were applied to the top of soil to mimic a field rate of 1120 ;kg NaCl/ha. Leachates (50 ;ml) from the pots were collected four times over 2 ;months. The leachates were concentrated by rotary evaporation and frozen. At the end of the experiment, plants were harvested, roots were rated for root rot and root colonization by placing roots onto selective media. Roots grown in noninfested soils were freeze-dried and analysed for mineral composition and for total nonstructual carbohydrates, amino acids, organic acids and total phenols. The leachates were thawed, and passed through ion-exchange columns to separate the neutral sugars, amino acids and organic acids. Sugars, amino acids, organic acids and total phenolic components were determined using colorimetric tests.

Results and conclusions
Root systems of NaCl-treated plants were larger, had fewer colonies of Fusarium pathogens per cm root, and had lower disease severity ratings than roots of untreated plants. The roots and the leachates from NaCl-treated pots contained more sugars and phenolic compounds, but less amino acids and organic acids than untreated roots. Roots treated with NaCl contained more Na, Cl, Mn and Zn than untreated roots. The findings suggest that NaCl may affect the mineral and organic content of the root and may influence the release of exudates by the roots. These changes may increase the resistance of the root to infection by at least two mechanisms, which are not mutually exclusive. First, the NaCl-mediated changes in the root exudates may selectively favour a disease-suppressive community of microbes, such as the Mn-reducing, pseudomonads. Second, these alterations in the exudates may impart a fungistatic effect on the pathogens before and during infection. The hypothesis that specific components in root exudates can affect some microbes, but not others will be discussed. Since asparagus presumably evolved in brackish inlets where exposure to salt would be common, the application of NaCl may simulate a natural soil environment where beneficial communities of microflora may be allowed to proliferate at the expense of harmful microbes.

1. Elmer WH,1992. Phytopathology 82, 97-104.
2. Elmer WH, 1995. Phytopathology 85, 1461-1467.