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VII ESA Congress, Cordoba, Spain, 15-18 July 2002

Manici L.M., Babini V., Caputo F.
Research Institute for Industrial Crops. Via di Corticella 133- 40129 Bologna (ITALY)
In recent years, interest has increased in the use of cover crops to enhance soil fertility (Elmer and
LaMondia, 1999). This practice stimulates the increase in soil microbial biomass which can
suppress soil-borne pathogens by competing for organic C availability. Many authors, have
however observed a significant increase in Pythium spp. in soil amended with fresh plant tissues.
Many Pythium species cause yield losses for crops worldwide. Pythium root rot is often associated
with soil management practices that increase crop residues in soil, such as direct drilling or green
manure (Cook et al., 1980). In fact, Pythium spp. has a high ability to survive as a saprophyte in
cultivated soil and cultural techniques can easily alter the interaction Pythium-microbial
community. Soil subject to intensive cultivation, requiring amendment with organic matter, is the
most frequently affected by unbalanced Pythium-microflora conditions. A study was carried out
under field and controlled conditions to evaluate Pythium and total fungal response after plant
tissue incorporation in soil with a high natural Pythium spp. population. This soil was chosen for
its high pathogenic Pythium spp. population (mainly P. ultimum and P. deliense) following several
years of continuous strawberry cultivation.
The field experiment was performed in the Cesena area (eastern Po Valley) in 1998, on a silty clay
loam soil (39% clay, 49% silt, 12% sand), naturally infected with Pythium spp. Barley (spring
sowing) green manure was compared with untreated soil. The trial was organized in a randomized
block design with four replicates (10X12 m). Soil samples (five soil cores, 0-20 cm, in each plot)
were collected from March to September, air dried for one week and sieved through a 5-mm mesh
Pythium spp and total fungal populations were recorded with the soil dilution plate method on
selective media: PARP (cornmeal amended with 5 mg l Pimaricin, 250 mg l Ampicillin, 10 mg Rifampicin, 100 mg PCNB and 1 g l Ox-gall) and water agar + 3 g l Ox-gall and 200 mg l
streptomycin sulfate. After incubation, the colonies were counted and expressed as Colony
Forming Units (CFU) g-1 soil.
A separate trial was performed in pots with soil from the trial field. Above-ground plant tissues
were collected at green manure time, chopped in a razor blender, and incorporated into the soil in
the pot. The plant biomass used was a realistic field rate (30 g kg-1 of soil). The trial was
organized in a randomized design with three replicates (1 pot=1 replicate). Pythium and total fungi
were recorded as above at four sampling times.
Pythium spp. and total fungal populations grew rapidly after biomass incorporation into the soil
and then decreased with the reduction in the availability of organic C. This response was observed
both in field and pot experiment (Fig. 1 and 2).
Pythium growth response to fresh biomass incorporation was faster than total fungi. In the field
trial, where Pythium and total fungi have a variable trend during the year, the Pythium population
reached its peak within 45 days and then rapidly decreased while total fungi began to increase
later than Pythium (Fig 1).
VII ESA Congress, Cordoba, Spain, 15-18 July 2002
Fig. 1 In soil Pythium and total fungi response
The simulation in pot gave the same Pythium to green manure in full field.
and total fungal response to fresh tissue Pythium spp.
incorporation, but to a much higher extent. This was probably due to the controlled conditions: fine plant tissue grinding, weekly watering, sieved, and consequently, more aerated soil Discussion.
Green manure is a common practice in organic farming, to maintain organic matter in soil and improve physical and nutritional soil characteristics. Depending on the existing conditions, saprophytic soil-borne pathogens in Total fungi
soil can be either actively suppressed by organic amendments (Lumsden at al. 1983) or enhanced. This can be dangerous when the soil- borne population is selected with previous soil results reported here show that 3 – 12 weeks after green manure Pythium populations increased. When the pathogenic Pythium population is high, total fungi, an important component of biomass, are unable to suppress Fig. 2 In pot response of Pythium and total
them as observed by several authors under fungi to fresh matter incorporation
other experimental conditions (Erhart et al., Pythium spp.
1999). The rapidity of Pythium response indicates that this pathogen can be particularly dangerous during the first weeks after green -1 so
Pythium and total fungal response to available organic C suggests several considerations for a. organic debris soil incorporation must take treatm ent 3 w eeks
6 w eeks 10 w eeks
into account previous soil management (monoculture, crop rotation). Total fungi
b. Unbalanced soils, with Pythium population selected by monoculture or short rotation, require a reduction in pathogenic population by appropriate crop rotation before using green c. Saprophytic pathogens and total fungi are strongly enhanced by plant tissue incorporation. Whether this can have a negative or positive effect on soil microflora, depending on the soil treatment 3 w eeks
6 w eeks 10 w eeks
Cook RJ, JW Sitton, & JT Walder. 1980. Plant Dis. 64: 102-103
Elmer WH & JA. LaMondia. 1999. Plant Dis. 83: 119-123.
Lumsden RD, JA Lewis & CG Papavizas. Environment. Sound. Agricult. (51-57). Praeg Scent.
Erhart E., K. Burian, W. Harti & K. Stik. 1999. J. Phytopathology 147: 299-305.


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