M. Avilés

Predictive factors for the suppression of Fusarium wilt of tomato in plant growth media

ABSTRACT Fusarium wilts are economically important diseases for which there are no effective chemical control measures. However, biological control and fertility management are becoming efficient alternatives for controlling this disease. Growth media formulated with composts that are able to suppress Fusarium wilt of tomato provide a control system that integrates both strategies. The aim of this study was to predict Fusarium wilt suppression of growth media using abiotic and biotic variables. Grape marc compost was the most effective medium used to suppress Fusarium wilt. Cork compost was intermediate, and light peat and expanded vermiculite were the most conducive growth media. The growth media evaluated were in a pH range of 6.26 to 7.97. Both composts had high beta-glucosidase activity. When pH and beta-glucosidase activity were taken into account as predictive variables, more than 91% of the variation in severity of Fusarium wilt was explained. This relationship illustrates the effect of nutrient availability and the degree of microbiostasis, two key factors in this pathosystem. Microbial populations involved in suppressiveness were cellulolytic and oligotrophic actinomycetes, fungi, and the ratios cellulolytic actinomycetes/cellulolytic bacteria, oligotrophic bacteria/copiotrophic bacteria, and oligotrophic actinomycetes/oligotrophic bacteria. Based on community level physiological profiles, different community structures were evident among growth media evaluated.

Trichoderma asperellum Strain T34 Controls Fusarium Wilt Disease in Tomato Plants in Soilless Culture Through Competition for Iron

Trichoderma asperellum strain T34 has been reported to control the disease caused by Fusarium oxysporum f.sp. lycopersici (Fol) on tomato plants. To study the importance of iron concentration in the growth media for the activity and competitiveness of T34 and the pathogen, we tested four iron concentrations in the nutrient solution [1, 10, 100, and 1000 µM provided as EDTA/Fe(III)] in a biological control experiment with T34 and Fol in tomato plants. The reduction of the Fusarium-infected shoot by T34 was only significant at 10 µM Fe. We hypothesized that Fe competition is one of the key factors in the biocontrol activity exerted by T34 against Fol, as an increase in Fe concentration over 10 µM would lead to the suppression of T34 siderophore synthesis and thus inhibition of Fe competition with Fol. T34 significantly reduced the populations of Fol at all the doses of Fe assayed. In contrast, Fol enhanced the populations of T34 at 1 and 10 µM Fe. Nevertheless, several plant physiological parameters like net CO2 assimilation (A), stomatal conductance (gs), relative quantum efficiency of PSII (ΦPSII), and efficiency of excitation energy capture by open PSII reactive centers (Fv′/Fm′) demonstrated the protection against Fol damage by treatment with T34 at 100 µM Fe. The first physiological parameter affected by the disease progression was gs. Plant dry weight was decreased by Fe toxicity at 100 and 1,000 µM. T34-treated plants had significantly greater heights and dry weights than control plants at 1,000 µM Fe, even though T34 did not reduce the Fe content in leaves or stems. Furthermore, T34 enhanced plant height even at the optimal Fe concentration (10 µM) compared to control plants. In conclusion, T. asperellum strain T34 protected tomato plants from both biotic (Fusarium wilt disease) and abiotic stress [Fe(III) toxic effects].

The suppressive effects of composts used as growth media against Botrytis cinerea in cucumber plants

The incidence/severity of soil-borne plant diseases is often reduced when composts are used as growth media. However, much less information is available about the effects of composts on the development of foliar diseases. Here we studied the suppressive capacity of five composts (from olive marc-cotton gin trash, grape marc, cork, spent mushroom and municipal organic and yard wastes) as growth media against Botrytis cinerea disease in cucumber plants. We also examined the putative correlations of several biotic and abiotic factors involved in disease suppression. The suppressive capacity of the growth media was studied by comparing disease incidence/severity in plants grown in composts with that occurring in plants grown in commercial peats, which are conducive to most soilborne diseases. Correlations were made between the occurrence of disease and leaf nutrient status, as well as electrical conductivity (EC) and microbial activity (measured as β-glucosidase activity) in the growth media. Cucumber plants grown in the peats showed greater severity of B. cinerea during the bioassay than those grown in composts. Mo, Ca and Si content in leaves showed negative correlations with this disease. A negative correlation with disease severity was observed for EC and microbial activity in the growth media. The noticeable reduction in B. cinerea in plants grown in composts was related to the supply of specific chemical elements, a certain degree of salt stress, and the high microbial activity of composts.

Effect of Trichoderma asperellum strain T34 on iron, copper, manganese, and zinc uptake by wheat grown on a calcareous medium

Rhizosphere microbes may enhance nutrient uptake by plants. Here we studied the effect of Trichoderma asperellum inoculation on the uptake of Fe, Cu, Mn, and Zn by wheat (Triticum aestivum L) grown in a calcareous medium. To this end, an experiment involving two factors, namely Fe enrichment (ferrihydrite enrichment and non-enrichment of the growing medium), and inoculation/non-inoculation with Trichoderma asperellum strain T34, was performed twice under the same conditions. The increase in Fe availability as a result of ferrihydrite enrichment did not enhance plant dry matter production. The effect of T34 on the concentration of Fe, Cu, Mn and Zn, and the total amount of Cu, Mn, and Zn in the aerial parts differed depending on the degree of ferrihydrite enrichment. Inoculation with T34 increased Fe concentration in Fe-deficient media, thus revealing a positive effect of this microorganism on Fe nutrition in wheat. However, T34 significantly decreased the concentration and total amount of Cu, Mn, and Zn in the aerial parts, but only in ferrihydrite-enriched medium. This adverse effect of T34 on Cu, Mn, and Zn uptake by wheat plants may have been related to conditions of restricted availability where potential competition for nutrients between microorganisms and plants can be more marked.

Carnation Fusarium wilt suppression in four composts

Fusarium wilt is now a major disease of carnation crops worldwide. Methyl bromide, which is used to remedy it, is environmentally unsafe. An alternative approach integrated into biological control is to grow crops in suppressive media. Suppressiveness of seven plant growth media to Fusarium oxysporum f. sp. dianthi was evaluated in bioassays with carnation (Dianthus cariophyllus) cv. Medea. These media were: (1) grape marc compost, (2) cork compost, (3) olive oil husk + cotton gin trash composted and mixed with rice husk, (4) spent mushroom compost mixed with peat, (5) coir fibre, (6) light peat and (7) vermiculite. In order to look for carnation Fusarium wilt suppressiveness indicators, growth medium pH and β-glucosidase activity were evaluated. Furthermore, F. oxysporum populations were measured in plant growth media at the beginning and end of bioassays. The compost media showed a range of suppressiveness in comparison with peat. Grape marc compost was the most effective plant growth medium in suppressing carnation Fusarium wilt. On the other hand coir fibre, peat and vermiculite were conducive for this disease. β-glucosidase activity and pH were positively correlated with disease severity as in other reports for tomato. Therefore, these two parameters are good indicators for carnation Fusarium wilt suppressiveness, and possibly for other F. oxysporum pathosystems. All composts showed similar F. oxysporum populations at the end of the bioassays to peat and vermiculite.

Grape Marc Compost Tea Suppressiveness to Plant Pathogenic Fungi: Role of Siderophores

It is important to know about the mechanisms that suppress plant diseases when compost from vegetable residues and/or their liquid extracts (compost tea) are used in order to improve the efficiency of this suppressing effect on pathogens. In this study, we assessed the presence of siderophores in various grape marc aerated compost teas (ACT) and their suppressing effect on nine pathogens: Rhizoctonia solani, Fusarium oxysporum f. sp. radicis-lycopersici, Fusarium oxysporum f. sp. lycopersici race 0, Fusarium oxysporum f. sp. lycopersici race 1, Fusarium oxysporum f. sp. radicis-cucumerinum, Verticillium dahliae, Pythium aphanidermatum, Phytophthora parasitica and the mycopathogen, Verticillium fungicola. Three concentrations (5, 10 and 15%) filtered, microfiltered and sterilized ACT were added to Petri dishes with a PDA medium, and 1 mM of ferric chloride (FeCy. After adding this mixture, a 0.5 cm disc was placed at the center of each dish containing the vegetative and reproductive body of each of the fungi to be tested. All the dishes were incubated at 25°C for seven days, except R. solani y P. aphanidermatum, which developed after 4 days. The addition of 1 mM of FeCl3 deactivated the siderophores present in the ACT, suppressing their inhibition of fungal development. The results obtained with the microfiltered ACT revealed that the microorganisms present in grape marc compost excreted siderophores into the medium which were responsible for inhibiting the growth of the 9 fungi tested. This activity was annulled by the addition of ferric chloride. The same results were achieved with the ACT obtained from filtering. This inhibition was not 100% after adding FeCl3 due to the fact that the microorganisms present in this tea, exhibited other biocontrol mechanisms.

First Report of Fusarium oxysporum f. sp. radicis-cucumerinum on Cucumber in Spain

During December 1999, root and stem rot was observed on greenhouse-grown cucumber (cvs. Albatros, Brunex, Acapulco, and Cerrucho) plants in Almería, Spain, using rock wool cultures. The disease caused severe damage, estimated at a loss of up to 75% of the plants, in the first greenhouse affected; afterward, the disease was found in eight additional greenhouses (14 ha) in 1999 and 2000. Stem lesions extended up to 10 to 12 cm above the crown in mature plants, although no fruit damage was observed. In the advanced stages, abundant development of orange sporodochia was evident on crown and stem lesions, without vascular discoloration. Root, crown, and stem pieces that were placed on potato dextrose agar (PDA) after surface-disinfection with 5% sodium hypochlorite, rinsed, and dried resulted in pure fungal colonies. Based on morphological characteristics of conidia, phialides, and chlamydospores from the isolations, the fungus was identified as Fusarium oxysporum Schlechtend.:Fr. Pathogenicity tests were conducted on cucumber (cvs. Marketmore 76 and Cerrucho [F1 hybrid]), melon (cvs. Amarillo oro, Perlita, Piboule, Tania, and Nipper [F1]), watermelon (cvs. Sugar Baby, Sweet Marvel, Jubilee, and Pata Negra and hybrid Crimson sweet), Cucurbita maxima × Cucurbita moschata, zucchini (cv. Senator), and loofah (Luffa aegyptiaca) at several stages: (i) pregermination; (ii) 1 or 2 true leaves; and (iii) more than 10 true leaves. Five fungal isolates were grown on PDA or shaken potato dextrose broth at 25°C for 8 days. Inoculation was performed in pots (10 seeds or plants of each cultivar or hybrid and isolate) by drenching with 100 ml of a fungal suspension (104 to 106 CFU/ml). Sterile water was applied to noninoculated control plants. Tests were repeated in growth chambers at 25°C (night) and 28°C (day) with a 16-h photoperiod. Fifteen to fifty days after inoculation, cucumber and melon plants at all three stages developed symptoms of root and crown rot in 100% of inoculated plants, with no observed vascular discoloration. Fifty days after inoculation, all three stages of C. maxima × C. moschata and zucchini remained symptomless. Loofah and watermelon germinated poorly or not at all when inoculated at the pregermination stage. Fifteen to fifty days after inoculation, 100% of inoculated cucumber and melon plants developed symptoms. Watermelon plants inoculated at the 10 or more true-leaf stage did not develop disease symptoms. No symptoms developed on noninoculated control plants. F. oxysporum was reisolated from infected roots, crowns, and stems of inoculated plants, confirming Kochs postulates. The main symptoms on cucumber infected by F. oxysporum f. sp. cucumerinum are wilt, yellowing, and vascular discoloration. In contrast, based on inoculation of the host differentials and the resulting disease symptoms found in this study, the fungus was identified as F. oxysporum f. sp. radicis-cucumerinum (1). To our knowledge, this is the first report of F. oxysporum f. sp. radicis-cucumerinum causing root and crown rot in cucumber in Spain. Reference: (1) D. J. Vakalounakis. Plant Dis. 80:313, 1996.

Selection of biological control agents against tomato Fusarium wilt and evaluation in greenhouse conditions of two selected agents in three growing media

Two biological control practices are the use of suppressive growing media and the application of biological control agents (BCAs). The goals of this study were: (i) to screen 584 potential BCAs obtained from Fusarium wilt (FW) suppressive growing media; (ii) to evaluate in greenhouse conditions selected BCAs in three growing media with different degrees of suppressiveness of tomato FW. Two isolates selected after screening were identified as Fusarium solani (305) and Streptomyces sp. (A19). Results showed that tomato FW was reduced and total production was improved when both BCAs were applied to a conducive medium (coir fiber). In highly suppressive growing medium (grape marc compost), A19 and 305 inoculations did not improve suppressiveness. In moderately suppressive growing medium (cork compost), only A19 improved this compost to natural grape marc compost suppressiveness level. Therefore, compost suppressiveness of tomato FW depended on the nature of the compost and on the isolates applied.

Identifying Characteristics of Verticillium Wilt Suppressiveness in Olive Mill Composts

The aims of this study were to assess the potential suppressive effects of different olive mill composts on Verticillium wilt and to elucidate the suppressive mechanisms. To this end, four olive mill composts from different crop areas with two maturation levels were selected. After conducting the Verticillium wilt bioassays in cotton, the suppressive effect was observed in only one compost. Compost maturation level did not affect disease development. The standardized area under the disease progress curve and microsclerotia concentration were associated with low API-ZYM enzymatic diversity, β-glucosidase activity, pH, and high electrical conductivity (EC). To assess the nature of suppressiveness in the suppressive compost, additional bioassays were performed with three treated compost-amended growing media (N-supplemented, autoclaved, and heat treated at 60°C for 6 days). Suppressiveness was partially reduced with heat treatments, where N-acetyl-β-glucosaminidase activity disappeared. In this compost, high oligotrophic actinomycete populations were associated with disease reduction. Therefore, plant growth media amended with different olive mill composts do not always show suppressiveness against Verticillium wilt. Enzymatic diversity, β-glucosidase activity, pH, and EC may be sufficient to predict where olive mill compost plant growth media will be effective in reducing Verticillium wilt and microsclerotia concentration. General and specific suppressiveness are involved in the mechanism of compost suppression.

Genetic Diversity of Fusarium oxysporum f. sp. dianthi in Southern Spain

The diversity of races and prevalence of pathogenic populations of Fusarium oxysporum f. sp. dianthi (Fod) were surveyed in an area in southern Spain. From 54 farms, 132 isolates were collected from wilted carnation plants. Isolates were characterized by RAPD-PCR, DNA sequence analysis of the TEF1-α gene, and race-specific molecular markers. Selected isolates from RAPD groups were phenotypically evaluated by pathogenicity tests. Data analysis showed that Fod race 2 was the most frequent and prevalent race in the study area, followed by race 1/8. Moreover, phylogenetic analyses showed similar results, which were different to those of the race-specific PCR assays. It was concluded that (i) seven isolates were not classified in groups where Fod testers were clustered; even they showed different results when race-specific markers were used, (ii) ten isolates with retarded race 1 or race 8 specific band were characterized as F. proliferatum by TEF1-α gene sequencing and clustered into an outgroup, and (iii) six isolates failed to generate an amplification signal using race-specific markers. Furthermore, three of them were grouped close to race 2 tester according to the phylogenetic analyses, showing the same differential pathogenicity as race 2. This may indicate a Fod race 2 subgroup in this region.