FUNGAL COMMUNITIES FROM THE RHIZOSPHERE OF TOMATO CULTIVATED CONVENTIONALLY AND WITH RYE AS COVER CROP

Agnieszka Jamiołkowska¹, Anna Wagner^2
1 Department of Plant Protection and Quarantine, Faculty of Horticulture and Landscape Architecture, University of Life Sciences in Lublin, Poland
² Department of Plant Protection and Quarantine, University of Life Sciences in Lublin, Poland

ABSTRACT

Fungal communities from the rhizosphere of tomato grown conventionally and in rye mulch were analyzed in 1998-2000. Their compositions depended on the type of cultivation and development phase of plant. The frequency of Fusarium equiseti, Aureobasidium pullulans, Talaromyces flavus and Penicillium spp. was higher in the soil amended with rye. Trichoderma spp. populations were similar in both environments but more numerous in August than in June. Rye mulch significantly affected the number of Fusarium oxysporum f.sp. lycopersici. The number of its isolates observed in the soil with cover crop was significantly lower than in control field.

Key words: cover crop, rye, tomato, soil-borne fungi, Fusarium oxysporum f.sp. lycopersici.

INTRODUCTION

In Poland the interest in alternate methods of vegetables cultivation is increasing recently due to the necessity of developing sustainable agricultural systems reducing chemical control of weeds and plant pathogens [7]. One of these methods is using cover crops to grow vegetables without tillage what not only reduce production costs but also increase soil fertility and suppress weeds [1, 2].

Grass crops protect the soil against erosion and nutrient leaching. By improving water infiltration they are conducive to early cultivation of vegetables in temperate climate. Rye is one of grass crops recommended both as off-season crop and as mulch for many cash crops. Apart from improving soil proprieties, cover crops can play an important role in the control of soil-borne pathogens [3, 16, 18]. The aim of this work was to investigate fungal communities developing in soil environment of tomato grown conventionally and in rye mulch without tillage.

MATERIALS AND METHODS

The experiments were conducted in 1998-2000 in the Felin Experimental Farm (near Lublin) on loess soil. Tomato plants of cv. Rumba were grown in two ways: conventionally (control) and with rye mulch without tillage on the plots of 100 m² each. Rye of cv. Dankowskie was sown the previous autumn and in the spring, when plants were 50 cm high, it was treated with Roundup 360 SL (1444 g·ha^-1). After two weeks, when dry mulch covered fully the surface, tomato seedlings were planted directly into the mulch. Control field was prepared in the traditional way. In 2^nd and 5^th weeks tomato plants were fertilized with Norway and lime salpetre (50 kg·ha^-1).

Soil from tomato rhizosphere was sampled twice a year (1^st week of June and 3^rd week of August) according to the method described by Lemańczyk and Sadowski [12]. Fungi isolation was carried out using Warcup’s plate method modified by Mańka [13]. Plates (50 for each combination) containing about 25 mg of soil-sand mixture were filled with Martin medium [14], incubated for 5 days and then the colonies of fungi were transferred into the tubes with PDA slants. Fungi growing from soil were identified with mycological monographs.

RESULTS

Over three years of investigations, in June 697 fungal isolates were obtained from rye mulch field and 614 from control field. This compares to 1073 isolates in August from cover crop field and 645 from control field in August (tab. 1, 2). Fungal communities isolated from the soil with rye mulch contained more species than those from the control field. The composition of communities differed also between June and August although some species were common in both seasons. The highest number of colonies belonged to Fusarium spp., Penicillium spp., Trichoderma spp., Aureobasidium pullulans and Talaromyces flavus. Among pathogenic fungi, F. oxysporum f.sp. lycopersici was isolated from all soil samples. However, the population of this pathogen in soil amended with rye mulch was less numerous (1.4-1.5% of all isolates) than in soil from control field (6.4-12.6%). The pathogen was isolated most frequently in August 1999 from control field, while at the same time its population was the smallest in the soil amended with rye mulch. F. solani occurred in control field in all years of investigations but from the field with cover crop that fungus was isolated only in 1999. F. culmorum was obtained more frequently only in June 1998 from rye mulch field. In other fungal communities it occurred sporadically. Non pathogenic to tomato F. equiseti predominated in August 1998 in both combinations but in other years that species occurred only in the rhizosphere of tomato grown with cover crop (tab. 1, 2).

Saprobiotic fungi predominated in all fungal communities. Penicillium spp. occurred in all soil samples but they were isolated more frequently from rye mulch field, especially in 1999. The populations of these fungi in the soil from cover crop field was generally similar in both seasons even if there were differences in species composition, while in control field Penicillium spp. were less numerous in August (with the exception of P. cyclopium in 1998). The frequency and species composition of Trichoderma spp. differed from year to year but generally the populations of these fungi were similar in both combinations. However, at earlier phase of tomato growth, more Trichoderma isolates were collected from rye mulch field. Aureobasidium pullulans amounted to similar percentage of all isolates in both investigated soils in June but in August that species was isolated only from cover crop field in 1999 and 2000. In the soil amended with rye, T. flavus populations were numerous in June 1999 as well as in August 1998 and 2000 while in control field it occurred only in June 1999. Regarding other saprobiotic fungi, the differences in their numbers were noticed between seasons, years and types of cultivation.

DISCUSSION

The research showed the influence of type of cultivation on fungal communities in tomato rhizosphere. Fungal communities from the soil under tomato grown in rye mulch were more diversified but some fungi occurred every year in both fields, particularly Fusarium spp., Penicillium spp., Trichoderma spp., A. pullulans and T. flavus. Those fungi are typical colonizers of soil environment of many crops, including tomato [11, 19]. However, the frequency of F. oxysporum f. sp. lycopersici was significantly lower in the rhizosphere of tomato grown in rye mulch. Rye could influence soil mycoflora both as living crop and as mulch. Root exudates of rye contain some compounds that can directly induce plant resistance [4]. Rye residues and components produced during decay process are also nutrient basis for many soil-borne microorganisms, what results in the increase of their populations [8, 16]. As rye prevents leaching of nitrogen, it could be responsible for numerous colonies of nitrofilious Penicillium spp. [15, 17]. Some of these species are antagonistic to F. oxysporum [9, 11], therefore their presence in tomato rizosphere can reduce the population of pathogen both by competition and by antibiosis. The same conditions favor T. flavus, frequently isolated, particularly in summer, from the rhizosphere of tomato grown in rye mulch.

Species of Trichoderma were isolated more frequently in August, probably due to higher soil temperature [10]. Their ability to produce antibiotics is enhanced in higher temperature. However, the production and activity of antibiotics is decreasing with the increase of soil pH [6]. The pH of investigated soil amounted to 5.5, what together with lower temperature in spring could have an adverse effect on Trichoderma activity in the rhizosphere. Numerous isolates of A. pullulans in rye mulch field are due to earlier colonization of rye roots [19]. F. equiseti is also related to cereals soil environment [12], therefore the numerous population of this fungus in rye amended soil is easy to explain. The presence of F. equiseti, a plant growth promoting fungus, can indicate an inhibitory effect of rye mulch. Horinouchi and Hyakumachi [5] proved a suppressive effect of this species on F. oxysporum f. sp. lycopersici.

CONCLUSIONS

1. Effect of rye, as cover crop on the population of F. oxysporum in tomato rhizosphere was significant. The number of F. oxysporum isolates collected from this environment was significantly lower than those obtained from the soil under tomato grown conventionally.

2. The populations of F. equiseti, Penicillium spp., A. pullulans and T. flavous were more numerous in the soil amended with rye mulch.

3. Trichoderma spp. colonized the rhizosphere of tomato every year irrespective of type of cultivation but their frequency was higher in August.

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