Wim J. Blok

Control of soilborne plant pathogens by incorporating fresh organic amendments followed by tarping.

ABSTRACT A new method for the control of soilborne plant pathogens was tested for its efficacy in two field experiments during two years. Plots were amended with fresh broccoli or grass (3.4 to 4.0 kg fresh weight m(-2)) or left nonamended, and covered with an airtight plastic cover (0.135 mm thick) or left noncovered. In plots amended with broccoli or grass and covered with plastic sheeting, anaerobic and strongly reducing soil conditions developed quickly, as indicated by rapid depletion of oxygen and a decrease in redox potential values to as low as -200 mV. After 15 weeks, survival of Fusarium oxysporum f. sp. asparagi, Rhizoctonia solani, and Verticillium dahliae in inoculum samples buried 15 cm deep was strongly reduced in amended, covered plots in both experiments. The pathogens were not or hardly inactivated in amended, noncovered soil or nonamended, covered soil. The latter indicates that thermal inactivation due to increased soil temperatures under the plastic cover was not involved in pathogen inactivation. The results show the potential for this approach to control various soilborne pathogens and that it may serve as an alternative to chemical soil disinfestation for high-value crops under conditions where other alternatives, such as solarization or soil flooding, are not effective or not feasible.

Long-Term Effect of Biological Soil Disinfestation on Verticillium Wilt

Biological soil disinfestation (BSD), involving incorporation of grass combined with plastic mulching, eliminates many soilborne pests and diseases through the creation of anaerobic conditions. BSD was compared at two locations with a nontreated control, Italian ryegrass amendment alone, and plastic mulch alone. After the soil treatments, plots were cropped with Acer platanoides and Catalpa bignonioides and grown for 4 years. Relative to the control, soil inoculum levels of Verticillium dahliae were reduced by 85% after BSD and did not increase for 4 years. Populations of Pratylenchus fallax, known for their interaction with V. dahliae, in the soil and in roots were reduced by 95 to 99%. The incidence of infection by V. dahliae was reduced by 80 to 90%. Verticillium wilt severity was significantly reduced in A. platanoides in all 4 years at one location and in the first 2 years at the other location, and significantly fewer plants died at one location. Shoot length and trunk width were larger after BSD compared with the control at one location. Market value of the crop in BSD plots was up to € 140,000 ha-1 higher for A. platanoides and up to € 190,000 ha-1 higher for C. bignonioides than in the untreated control. BSD is an effective, economically profitable, and environmentally friendly control method for tree nurseries.

Relation between soil health, wave-like fluctuations in microbial populations, and soil-borne plant disease management

A healthy soil is often defined as a stable soil system with high levels of biological diversity and activity, internal nutrient cycling, and resilience to disturbance. This implies that microbial fluctuations after a disturbance would dampen more quickly in a healthy than in a chronically damaged and biologically impoverished soil. Soil could be disturbed by various processes, for example addition of a nutrient source, tillage, or drying-rewetting. As a result of any disturbance, the numbers of heterotrophic bacteria and of individual species start to oscillate, both in time and space. The oscillations appear as moving waves along the path of a moving nutrient source such as a root tip. The phase and period for different trophic groups and species of bacteria may be shifted indicating that succession occurs. DGGE, Biolog and FAME analysis of subsequent populations in oscillation have confirmed that there is a cyclic succession in microbial communities. Microbial diversity oscillates in opposite direction from oscillations in microbial populations. In a healthy soil, the amplitudes of these oscillations will be small, but the background levels of microbial diversity and activity are high, so that soil-borne diseases will face more competitors and antagonists. However, soil-borne pathogens and antagonists alike will fluctuate in time and space as a result of growing plant roots and other disturbances, and the periods and phases of the oscillations may vary. As a consequence, biological control by members of a single trophic group or species may never be complete, as pathogens will encounter varying populations of the biocontrol agent on the root surface. A mixture of different trophic groups may provide more complete biological control because peaks of different trophic groups occur at subsequent locations along a root. Alternatively, regular addition of soil organic matter may increase background levels of microbial activity, increase nutrient cycling, lower the concentrations of easily available nutrient sources, increase microbial diversity, and enhance natural disease suppression.

Fusarium redolens f.sp. asparagi, causal agent of asparagus root rot, crown rot and spear rot.

Two Fusarium species, F. oxysporum f.sp. asparagi and F. proliferatum, are known to be involved in the root and crown rot complex of asparagus. We have investigated reports on the involvement of F. redolens, a third species, which until recently was considered conspecific with F. oxysporum because of morphological similarities. RFLP analysis of the rDNA internal transcribed spacer region and AFLP fingerprinting identified eight strains from asparagus unambiguously as F. redolens. Four of these were tested and found to be pathogenic to asparagus either in this study (two strains) or in a previous one in which they were classified as F. oxysporum (three strains). Disease symptoms and disease development were the same as with F. oxysporum f.sp. asparagi and F. proliferatum. Present data and literature reports identify F. redolens as a host-specific pathogen involved in root, crown and spear rot of asparagus. The pathogen is formally classified as F. redolens Wollenw. f.sp. asparagi Baayen.

Differences in N uptake and fruit quality between organically and conventionally grown greenhouse tomatoes

Soil-bound intensive greenhouse production has been scrutinized for its sustainability due to contamination of ground water by over-fertilization resulting in leaching of nutrients. As environmental guidelines are becoming more restrictive worldwide, and especially in Europe, many greenhouse growers have converted to more sustainable production systems including rockwool culture with recycled water and organic cropping systems in soil. The increase in popularity of organic production systems has amplified the debate whether organically grown produce is healthier than conventional produce. So far, little is known about the variations in fruit quality associated with production systems for greenhouse grown tomatoes. Thus, two organic (organic fertilization with and without straw amendment) and three conventional tomato cropping systems (regular and increased nutrient solution in rockwool and regular fertilization in soil) were compared in order to evaluate differences in nutrient availability and effects on fruit quality over a three-year period. Three modern medium-sized round tomato cultivars and one old cultivar were compared. There were no significant interactions between cropping systems and cultivars, so that main effects of systems and cultivars could be evaluated. Fruit yields in the organic systems were similar to those obtained in the conventional soil-bound system, but 15% lower than in the regular rockwool system, even though nitrogen concentrations in soil were not limiting in any of the production systems. Frequent organic amendments resulted in higher soil NO32− contents in the organic system without straw than in the other soil-bound systems, indicating that the organic systems were not yet stable in terms of nutrient availability after three years. A fruit quality index, based on the contents of compounds such as lycopene, β-carotene and vitamin C, was similar in all cropping systems. The old cultivar had a significantly higher quality index, but a lower yield than the other cultivars. According to this study, high quality tomatoes can be obtained through proper adjustment of the quantity and the source of nitrogen fertilizers in organic and conventional cropping systems and the use of selected cultivars with a high nutrient use efficiency for organic systems.

Daily Changes of Infections by Pythium ultimum After a Nutrient Impulse in Organic Versus Conventional Soils

Bacterial populations (CFU) have been shown to oscillate in wavelike patterns after nutrient impulses in previous studies. The amplitudes and periods of oscillations could possibly be used as indicators of soil health analogous to the stability and resilience of biological populations widely accepted as indicators for ecosystem health. Limited plant and animal disease outbreaks can also be viewed as a manifestation of a healthy soil ecosystem. Two pot experiments were carried out to verify whether damping-off of beet seedlings by Pythium ultimum, measured as area under the disease progress curve (AUDPC), fluctuated over time after incorporation of organic materials into organic versus conventional soils, and to investigate whether daily dynamics of AUDPCs were linked to the dynamics of microbial populations and chemical parameters. AUDPCs oscillated significantly over time when Pythium bioassays were initiated daily after addition of ground grass and clover shoots (GC) into unplanted soils. Similar oscillations with significant harmonics of AUDPC were also observed in composted manure (CM)-amended soils but with smaller amplitudes than in GC-amended soils. The AUDPC harmonics in amended soils had periods similar to those of CFU of copiotrophic bacteria. Cross-correlation analysis demonstrated that periodic fluctuations of P. ultimum infections (AUDPCs) did not coincide with those of copiotrophic CFU but were shifted in phase. It appears that competition or antagonism from some fast-growing bacteria influenced pathogen infections, because these bacterial populations were growing and dying. Soil chemical variables, including pH, dissolved organic carbon, and NO(3)(-)-N, and NH(4)(+)-N contents, changed significantly in the initial 7 days after a nutrient impulse into soils. These changes were cross-correlated with copiotrophic CFU with time lags of approximately 1 to 2 days but were seldom associated with daily changes in AUDPCs. Organically managed soils always had lower AUDPC ratios of amended to nonamended treatments, indicating that organic materials showed stronger suppressive abilities to P. ultimum in organic than in conventional soils. The oscillations in AUDPCs and copiotrophic CFU in amended organic soil also had smaller amplitudes than in amended conventional soil. These results suggested that organically managed soils had a greater resistance and resilience to the disturbance of the amendments and, therefore, could be considered healthier than conventionally managed soils.

Phytosanitary risk assessment of composts

Assessment of phytosanitary risks associated with application of composts in agriculture generally has focused on the sanitation (self-heating) phase during composting when most plant pathogens are inactivated due to lethal temperatures. However, a few plant pathogens are heat resistant and they may survive a properly monitored and controlled composting process. To assess the phytosanitary risks associated with compost utilization, several additional factors need to be considered which all relate to a tracing-and-tracking principle. It includes the composition of the original waste and several aspects related to compost utilization. The following parameters are considered to be key-factors: 1) the proportion of host biomass relative to the total quantity of biowaste, 2) the proportion of host infected with a pathogen, 3) the density of infected host material, 4) the proportion of propagules of a pathogen that survived the process, and 5) the threshold density of a pathogen in soil above which disease of the host is expected to develop. While the first two parameters may be rather easy to estimate and information on survival of many plant pathogens can be obtained from the literature, little knowledge exists on the density of the pathogens in host materials or on threshold values. This applies particularly to virus diseases. The phytosanitary risk of several types of plant pathogens is discussed in some detail in this paper. Recommendations are given for testing of a composting process for phytohygienic safety.