Efficacy of rue extract, sodium bicarbonate and fungicides at ... - Inea

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58Il Tabacco, 9industries and have broad-spectrum antimicrobialactivity. Moreover, they have been tested forcontrolling powdery mildew on several plantspecies (1,6,9,10,16).The objective of this study is to evaluate theefficacy of rue extract, sodium bicarbonate andfive fungicides at low rates in greenhouse andfield trials to control E. orontii on tobacco plants.Materials and MethodsFigure 1 - Conidiophores and conidia of Erysipheorontii.Fig. 1. Conidiofori e conidi di Erysiphe orontii.cells of the conidiophores and the conidia chainsare curved and with crenate edges in E. orontii(Fig. 1), while they are erect with sinuate edgesin E. cichoracearum (Fig. 2). Plotnikova et al.,(1998) identified E. orontii as the agent ofpowdery mildew even on Arabidopsis thaliana,where E. cichoracearum and E. cruciferarum hadbeen previously reported. Erysiphe isolatesrecovered from Italian tobacco crops correspondto the E. orontii description.Powdery mildew has spread to most countrieswhere tobacco is cropped, often occurring as adisease of major importance. When resistantcultivars are not available, fungicide applicationsare necessary to avoid yield losses, neverthelessthe reduction of agrochemicals use is one of themajor objectives to be achieved in Europeantobacco growing areas. Natural products,inorganic salts and/or fungicides at lower ratesthan those used on other crops, may limit bothresidues on cured tobacco leaves and theenvironmental impact of the crop. Biologicallyactive natural products (allelochemicals)isolated from higher plants are now beingemployed as fungicides and in biocontrol ofplant diseases (8,14).Oliva et al. (1999) demonstrated significantin vitro fungistatic activity of rue (Rutagraveolens L.) extract against six fungal species.Bicarbonate salts are widely used in the foodFungal isolate and inoculation: Plants withsymptoms of powdery mildew were harvested ina farm located in a typical «burley» tobaccogrowing area (Caserta) of southern Italy. Conidiafrom those plants were collected, then inoculatedon 2 month old plants using a suspension adjustedto 10-20 conidia/ml with the aid of anhemacytometer in 0.1% sterile water agar (SWA)to allow the development of spare colonies (Fig.3). At theoutbreak ofsymptoms,conidia from asingle spotwere re-inoculatedon theleaves of healthyplants. Thisprocedure wasrepeated until20 plants werewell colonizedas a source ofinoculum forthe greenhousetest.Suspensionconcentrationused for testingactive ingredientswas 10 5conidia/ml and10 ml/plantwas appliedusing a labo-Figure 2 - Conidiophores andconidia of Erisiphae cichoracearum.Fig. 2. Conidiofori e conidi diErisiphae cichoracearum.ue22ing.p65 5825/03/2005, 17.22
Lahoz et al. Efficacia dell’estratto di ruta, bicarbonato e....... 59ratory sprayer (Fig. 3). The inoculation was made1 day after the first application of the rue extract,sodium bicarbonate and fungicides.Greenhouse testing of active ingredients. Twotrials (A and B), planned as random completeblock design (RCBD) with four replicates, werecarried out at Tobacco Experimental Institute in1997 and 1998. Plants of S2/95 cultivar, whichis highly susceptible to Erysiphe orontii, weretransplanted in 35 cm plastic pots (1 plant/pot)after 30 days from sowing; treatments wereapplied to all leaves on 10, 3-month-old, plantsof each replicate. Temperature in the greenhousewas 22 ± 2°C with continuous illumination toallow sporulation (7) and 75 ± 10% relativehumidity. Rue extract, sodium bicarbonate andfive fungicides (Penconazole 10EC;Tebuconazole 25 WG; Benomyl 50WP; Dinocap25WP; Bupirimate 25 EC) at low rate ofapplication were compared. All treatments wereapplied with a laboratory hand sprayer to run off.Rates, intervals and number of applications foreach treatment are reported in Table 1.Phytotoxicity was evaluated using anempirical scale with values ranging from 1 (nophytotoxicity) to 9 (plant death).Disease severity (DS) was assessed using twomethods: a) starting from the outbreak ofsymptoms, the number of sporulating mildewcolonies on 8 basal leaves for each of 10 plants/replicate was recorded every 8 days; b) the leafarea covered with powdery mildew symptomswas evaluated 8 days after the last application,then percentage of disease control was calculated.Field trialTwo field trials planned as completely randomizedblock design with 4 replicates were carried out in1997 and 1998. Plants of cultivar S2/95 weretransplanted at 20/5 and 15/5 respectively for thetwo trials. Applications of the fungicides (the sameforeseen in the greenhouse test) started from thefirst ten days of July. The influence of the treatmentson the technological characteristics and on theefficacy were evaluated. Disease severity wasevaluated using Coresta scale ranging from 0 (nosymptoms) to 5 (> 50 % of leaf surface covered bythe fungus).Figure 3 – Leaves of inoculated tobacco plantsshowing spare colonies (top) and leaves inoculatedwith a higher concentration of conidia of Erysipheorontii.Fig. 3. Foglie di tabacco inoculate mostranti rare colonie(in alto) e foglie inoculate con elevate concentrazionidi conidi di Erysiphe orontii.Rue extract preparation. Fresh leaves of R.graveolens were collected from plants grown atthe Tobacco Research Institute in Scafati, Italy.250 g of fresh leaf material were soaked in 1 literdeionized water at 90 °C for 10 min, followedby filtration through Whatman No. 1 filter paper(17). The filtrate was lyophilized, yielding 10 g/l of powdered extract.Microscopic examination of conidia. Fourplants were inoculated with 10 ml (10 5 conidia/ml) of E. orontii conidial suspension, two of themwere then treated with rue extract and two withsterile distilled water (SDW). 24 h after theue22ing.p65 5925/03/2005, 17.22
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