Preliminary results on salicylic acid treatment on brown rot caused by Monilinia laxa on Jumbo Cot fruit , Prunus armeniaca

Brown rot, caused by Monilinia laxa (Aderhold &Ruhland) is a devastating disease of apricot (Prunus armeniaca L.). The disease is endemic in Europe and causes epidemics in most stone fruit orchards (Wormald, 1954; Byrde & Willets, 1977; Batra, 1991; Holb, 2004). Brown rot has been resulting in blossom blight in rainy springs and in fruit rot in rainy summers in Hungarian apricot orchards. Postharvest fungal pathogens result in major losses of fruits and vegetables and can be controlled effectively by synthetic chemical fungicides Many reports have shown that induced disease resistance in plants by biotic and abiotic elicitors is a very effective method for restricting the spread of fungal infection (Droby et al., 2002; Qin et al., 2003). Resistance of plants to pathogens is based on both constitutive defense mechanisms such as pre-existing antimicrobial compounds and inducible defense mechanisms. Induced disease resistance in plants by biotic or abiotic treatments is a very attractive strategy for controlling diseases This experiment aimed to study the effect of salicylic acid on the in vitro growth of M. laxa.


Introduction
Brown rot, caused by Monilinia laxa (Aderhold &Ruhland) is a devastating disease of apricot (Prunus armeniaca L.).The disease is endemic in Europe and causes epidemics in most stone fruit orchards (Wormald, 1954;Byrde & Willets, 1977;Batra, 1991;Holb, 2004).Brown rot has been resulting in blossom blight in rainy springs and in fruit rot in rainy summers in Hungarian apricot orchards.Postharvest fungal pathogens result in major losses of fruits and vegetables and can be controlled effectively by synthetic chemical fungicides Many reports have shown that induced disease resistance in plants by biotic and abiotic elicitors is a very effective method for restricting the spread of fungal infection (Droby et al., 2002;Qin et al., 2003).Resistance of plants to pathogens is based on both constitutive defense mechanisms such as pre-existing antimicrobial compounds and inducible defense mechanisms.Induced disease resistance in plants by biotic or abiotic treatments is a very attractive strategy for controlling diseases This experiment aimed to study the effect of salicylic acid on the in vitro growth of M. laxa.

Isolation of fungi
M. laxa were isolated from decayed plum fruit.The fungi were maintained on PDA at 4 o C. Spores of M. laxa were obtained from 2-week-old cultures incubated at 25 o C by fl ooding the cultures with sterile distilled water containing 0.05% (v/v) Tween 80.The suspensions of spores were fi ltered through four layers of sterilized cheese cloth.The concentrations of spores were adjusted to (1 × 10 -3 spores ml -1 ) with the aid of a haemocytometer

Mycelial growth
The effects of Salicylic acid on mycelial growth were assayed by the method of Yao & Tian (2005a).Salicylic acid solution mixed with molten PDA-agar to give a total volume of 20mL per petri plate (diameter: 90mm).S.A concentration was 2 Mm in the PDA-agar.After the agar had solidifi ed, 5mm disks of M. laxa were placed in the center of each petri plate.Plates were incubated at 20 o C.Colony diameter was determined 24, 48 and 72h after inoculation.Each treatment was replicated three times and the experiment was repeated twice.Mycelial growth of M. laxa on PDA was expressed as growth rate, which was calculated according to the following formula.Growth rate (%) = (colony diameter after inoculation−5mm)/5mm ×100.

Effect of salicylic acid treatment on mycelial growth of Monilinia laxa in vitro
Salicylic acid treatment markedly reduced the growth rate of M. laxa in vitro fi gure (1).Growth rate of M. laxa in PDA media without salicylic acid rose to around 50% during 2 days after incubation at 25 o C while the rate rose to about 20% in PDA media inoculate with 2Mm salicylic acid with signifi cantly differences (p<0.05).The growth rate of M. laxa in control treatment increased dramatically after 4 days and 6 days to around 90% and 140%, respectively.In case of salicylic acid treatment, the rate of mycelial growth ranged between approximately 25% to 45% after 4 days and 6 days, respectively with signifi cant difference in comparison to control treatment (p<0.05).
Inducing resistance is another important strategy for reducing the diseases in plant as well as in harvested fruit and vegetables, which is attractive because the induced disease resistance utilizes the plant's own defense mechanisms and is either far less toxic than fungicides or nontoxic (Wilson et al., 1994;Kuć, 2001;Terry and Joyce, 2004;Walters et al., 2005;Elmer and Reglinski, 2006).For example, salicylic acid (SA), a best known elicitor, can induce local and systemic resistance in many plant tissues (Rojo et al., 2003;Garcia-Brugger et al., 2006) including pear fruit (Cao et al., 2006;Tian et al., 2006).
In this study, found that salicylic acid treatment could signifi cantly reduce M. laxa growth in vivo as Cao et al. (2008) reported in case of MeJA in loquat fruit.It is postulated that the control of the disease is directly because of the inhibitory effect of MeJA on pathogen growth, and indirectly because of the induced disease resistance triggered by enhanced H 2 O 2 levels.Also these results were in the same way with Yao et al. (2005b), they reported that salicylic acid with a concentration of 2mM showed direct fungitoxicity on M. fructicola and signifi cantly inhibited mycelial growth and spore germination of the pathogen in vitro.The results reported that the Salicylic acid has direct effect on reducing the lesion diameter and disease incidence with signifi cantly differences (p<0.05) and this fi nding is too clear in case of unwounding treatment these results are in the same trend like the fi nding of Yao et al. (2005ab) they reported that preharvest treatments with 2mM salicylic acid (SA) and 0.2mM methyl jasmonate (MeJA) signifi cantly reduced lesion diameters on sweet cherry fruit caused by M. fructicola compared with control post-harvest treatments.

Figure 1 .
Figure 1.Effect of salicylic acid on mycelial growth of Monilinia laxa in vitro.Data are expressed as the mean of triplicate assays.Vertical bars represent the standard errors of means