versão On-line ISSN 0718-1620
Cienc. Inv. Agr. vol.39 no.1 Santiago abr. 2012
Cien. Inv. Agr. 39(1):221-228. 2012
Effect water activity on in vitro mycelial growth of Neofusicoccum spp. infecting blueberry
Efecto de la actividad del agua y temperatura sobre el crecimiento micelial in vitro de Neofusicoccum spp. de arándano
Bernardo A. Latorre, Gonzalo A. Díaz, and María P. Reed
Facultad de Agronomía e Ingeniería Forestal, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago Chile.
Stem canker in blueberries (Vaccinium corymbosum L.) is a disease which is widely distributed around the world and of growing importance in Chile, associated with Neofusicoccum species. The purpose of the present work was to study the effects of water activity (aw) and temperature on mycelial growth in vitro of N. arbuti, N. australe and N. parvum, obtained from blueberries with symptoms of stem canker. According to the results obtained, 25 °C was the optimum temperature for mycelial growth of N. arbuti, N. australe and N. parvum. These species presented minimal growth at 35 °C and none at 0 and 5 °C. The effect of aw on mycelial growth was significant (P≤0.001) and varied with the species of Neofusicoccum and the temperature. At 25 °C, the optimum temperature, the three species of Neofusicoccum presented minimum growth with aw of 0.900 and 0.920, while optimum mycelial growth was found with aw equal to 0.990. Independent of the Neofusicoccum species, the effect of temperature (T) and aw on mycelial growth was best explained by y = -721.62 + 0.24T + 788.92aw. (R2adj = 0.77, P<0.001), suggesting that T and aw jointly explained 77% of the total variation on mycelial growth of Neofusicoccum spp. The results obtained provide important information for a better understanding of the biology of these plant pathogenic fungi, and suggest that temperature and aw could be important parameters for understanding factors that affect the inoculum production, colonization and infection by Neofusicoccum spp. in blueberry plantations.
Key words: Ecological determinants, Neofusicoccum arbuti, Neofusicoccum australe, Neofusicoccum parvum, stem canker, dieback, Vaccinium, water potential.
La cancrosis de la madera en arándanos (Vaccinium corymbosum L.) es una enfermedad de amplia distribución en el mundo y de creciente importancia en Chile, asociada a la acción de especies de Neofusicoccum. Este trabajo tuvo como objetivo estudiar el efecto de la actividad de agua (aw) y de la temperatura sobre el crecimiento micelial in vitro de N. arbuti, N. australe y N. parvum, obtenidos de arándanos con síntomas de cancrosis de la madera. Según los resultados obtenidos, 25 °C fue la temperatura óptima para el crecimiento micelial de N. arbuti, N. australe y N.parvum. Estas especies crecieron mínimamente a 35 °C y no crecieron a 0 y 5 °C. La aw afectó significativamente (P≤0.001) el crecimiento micelial y su efecto varió en función de la especie de Neofusicoccum y de la temperatura. A 25 °C, temperatura óptima, las tres especies de Neofusicoccum crecieron mínimamente con aw de 0,900 y 0,920, siendo óptimo el crecimiento micelial con aw igual a 0,990. Independientemente de Neofusicoccum spp., el crecimiento micelial (y) entre 20 y 30 °C se explicó a través de y = -721,62 + 0,24T + 788,92aw. (R2 = 0,77, P<0,001). Los resultados obtenidos aporta información para la mejor compresión de la biología de estos hongos fitopatógenos y sugieren que la aw es un importante factor que podría afectar la producción de inóculo, colonización y la infección por Neofusicoccum spp., en arándanos.
Palabras clave: Actividad del agua, arándano, Botryosphaeriaceae, cancrosis, enfermedades, temperatura, Vaccinium.
The blueberries (Vaccinium corymbosum L.) are highly important in Chile. There are at present 10,763 ha planted with blueberries, distributed between Coquimbo (29°57' S) and Osorno (40°34'S), along a north-south axis of approximately 1,400 km, leading to great variations in climate and soil conditions (INE, 2010). Under these conditions, stem canker caused by species of genus Neofusico-ccum has acquired great economic importance. N. arbuti, N. australe, and N. parvum, have been associated with blueberry stem canker in Chile (Espinoza et al, 2009).
Most species in the family Botryosphaeriaceae have a wide host range and geographical distribution and they are favored by drought-stress (Desprez-Loustae et al, 2006; Ma et al, 2001). However, free moisture and high relative humidity appear to be necessary for conidial germination and infection of species of Botryosphaeriaceae (Arauz and Sutton, 1989; Sutton and Arauz, 1991). Optimum temperatures for germination of conidia and mycelial growth vary among species in the family Botryosphaeriaceae. For instance, the optimum temperature for mycelial growth of N. arbuti, N. australe and N. parvum was 25 °C and 30 °C for Botryosphaeria dothidea (Espinoza et al, 2009; Kohn and Hendrix, 1982). The optimum temperature for conidial germination of most Botryosphaeriaceae species has been found between 25 and 30 °C, but 40 °C for pigmented conidia of Lasiodiplodia theobromeae (Sutton and Arauz, 1991, Úrbez-Torres et al., 2010). Similar to other hosts, it has been postulated that pycnidia, formed on disease wood and pruning debris in blueberry plantations, release conidia during rainy periods and /or under high relative humidity conditions (van Niekerk et al, 2010).
Temperature and water activity (aw) are the most important abiotic parameters determining the potential for mycelial growth of fungal pathogen on the phyllosphere (Magan and Lacey, 1984), being aw the same value of relative humidity under equilibrium conditions. Knowledge of the thermal and humidity requirements necessary for spore germination and mycelial growth has been employed in the past in formulating predictive models. These models make it possible to predict favourable conditions for infection, and thus, to orientate chemical control of numerous diseases (Arauz and Sutton, 1989; Bendek et al, 2007; Broome et al., 1995; Latorre et al., 2002; Sutton and Arauz, 1991). The effect of temperature and aw on mycelial growth in species of Neofusicoccum which cause canker in blueberries has not been investigated. Nevertheless, it is considered that this information is necessary for a better understanding of the epidemiology of stem canker infection. The purpose of this work was therefore to study the effect of aw and temperature on the mycelial growth of three species of Neofusicoc-cum previously described in blueberries in Chile (Espinoza et al., 2009).
Materials and methods
The effect of temperature was studied in vitro in acidified potato dextrose agar (APDA) containing per liter 20 g agar, 20 g dehydrated potato, 20 g glucose and 0.5 mL 92% lactic acid, pH 4.2. The aw was determined in APDA amended with 0, 17.4, 35.5, 65.7, 115.7 and 142 g L-1 NaCl, equivalent to aw of 0.996, 0.990, 0.980, 0.960, 0.920 and 0.900, respectively (FDA, 2009).
The following isolates, obtained from blueberry stem cankers in Chile and previously identified, were used in this study: N. arbuti (isolate B1-09, Osorno, Chile), N. australe (isolate B1-05, Rapel, Chile) and N. parvum (isolate B1-06, Santiago, Chile) (Espinoza et al., 2009). These isolates were activated in APDA at 20 °C and maintained in the same culture medium at 5 °C.
The incubation chambers used were Velp Scientifica FOC 225E (Velp, Usmate, Italy). The inside temperature of the chambers was verified with HOBO PRO temperature sensors (Onset Computer Corp. Bourne, Massachusetts, USA) that were maintained for 5 days before starting the tests with a ± 0.2 °C error.
Effect of temperature
The effect of temperature on mycelial growth of Neofusicoccum spp. was previously reported (Espinoza et al., 2009). However, prior to study the effect of aw, the optimal range of temperature for mycelia growth in vitro was reassessed in this study. With this purpose, a piece of mycelium (5 mm diameter), obtained from pure cultures of 7 days in APDA, was placed in the centre of a Petri dish with APDA and incubated at 0, 5, 10, 15, 20, 25, 30 and 35 °C (±0.2°C). The diameters of the colonies developed were determined at 48 h of incubation.
Effect of water activity (aw)
The effect of aw on mycelial growth in vitro was studied at 20, 25 and 30 °C. With this purpose, a plug of agar (5 mm diameter) taken from 7 day old cultures of each isolate in APDA at 20°C, was placed in the centre of a Petri dish. The APDA medium used was amended with 0, 17.4, 35.5, 65.7, 115.7 and 142 g L-1 NaCl, resulting in aw of 0.996, 0.990, 0.980, 0.960, 0.920 and 0.900, respectively (FDA, 2009). The mycelial growth was determined at 48 and 96 h of incubation.
Design and statistical analysis
In the first test, the effect of temperature on mycelial growth was studied as a completely randomized design with a 3x8 (Neofusicoccum species x temperature) factorial structure with three replicates for each species and each temperature treatment. In the second test, the effects of the Neofusicoccum species, temperature and aw, were distributed in a completely randomized design, with a 3x3x6 (species x temperature x aw) factorial structure, replicated three times for each Neofusicoccum species, aw and temperature.
The results were subjected to analysis of variance and means were separated using Tukey'sawphairwise comparison test (P≤0.05) using the SigmaStat statistical programme (Systat Software, Inc., USA). The relationship among mycelial growth, temperature and aw was studied with a multiple linear regression analysis in which the dependent variable was mycelial growth and the independent variables were temperature and aw. In addition, the relation between a and mycelial growth was determined by polynomial regression analysis between aw = water activity and y = mycelial growth determined at 96 h at 25 °C. The models y = f (aw) and f (aw, T) were tested to observe the relationship between the effect of water activity (aw) and/or temperature (T) on the mycelial growth (y), using data pooled from three replicates. Regression analysis was used to estimate these parameters. Data were fitted to the model by coefficient of determination (R 2) using SigmaStat statistical programme.
Effect of temperature
Neofusicoccum spp. grew at temperatures between 10 and 35 °C. There was no growth at 0 and 5 °C (Figure 1). The optimum temperature for mycelial growth was 25 °C. At this temperature, N.parvum had the greatest radial growth of the mycelium with 72.7 mm, while N. australe and N. arbuti had an average mycelial growth of 61.3 and 37.5 mm, respectively. The temperature and Neofusicoccum spp. had a significant effect (P≤0.001) on mycelial growth. Likewise the interaction between species and temperature was significant (P≤0.001). The mean differences in mycelial growth obtained between the Neofusicoccum species were statistically significant (P=0.05) (Table 1).
Effect ofwater activity (aw)
All the Neofusicoccum species grew in media with aw between 0.900 and 0.996. Independent of temperature, N. parvum attained optimum mycelial growth at aw of 0.996, while N. australe and N. arbuti attained optimum mycelial growth at aw of 0.990 (Figure 2). The species, temperature and aw had a significant (P≤0.05) effect on mycelial growth, with significant (P<0.027) interactions between Neofusicoccum species (N) and temperature (T), between N and aw, T and aw, and between N, T and aw (Table 1).
The relation between the aw at 25 °C and mycelial growth was explained by polynomial models where y = 14917 aw 2 - 27480 aw + 12657 (R2 = 0.95) for N. arbuti y = 13036 aw2 - 23889 aw + 10950 (R2 = 0.93) for N. australe, and y = 15763 aw2 - 29087 aw + 13417 (R2 = 0.97) for N.parvum (Figure 3). Independently of Neofusicoccum species, the mycelial growth (y) was modelled by the following quadratic function y = -721.62 + 0.24T + 788.92aw , (R2 d = 0.77, P≤0.001), where T = temperature and aw = water activity. Standard errors were: 0.52, 59.08 and 15.62 for T, aw and for the estimates, respectively.
Several species in the family Botryosphaeriaceae have been associated with stem cankers of blueberries. In Chile, N. arbuti, N. australe and N. parvum were reported (Espinoza et al., 2009), but it is possible that they may coexist with other species of the Botryosphaeriaceae family and other plant pathogenic fungi. For example, Pestalotiopsis clavispora, P. neglecta and Truncatella angustata were frequently found associated with stem canker in blueberries (Espinoza et al., 2008).
This study has shown mycelial growth of Neofusicoccum spp. can be reduced, but not completed inhibited, by lowering the aw, suggesting that this factor may play an important role in the blueberry infection and colonization of woody tissues under field conditions. High water stress has been reported to enhance canker formation and colonization by species in the family Botryosphaeriaceae (Crist and Schoeneweiss, 1975; Desprez-Loustau et al., 2006; Ma et. al., 2001; Magar et al., 1989). In addition, the influence of aw and temperature on fungal growth is of great importance in understanding the ecological relations between species of fungus and to understand the pathogenesis of wood colonizer fungi (Desprez-Loustau et al., 2006; Ma et al., 2001; Valík and Piecková, 2001). Therefore, the results obtained in this study suggest that aw could be an important parameter for understand the critical factors affecting the production of inoculum and mycelial colonization of woody tissues in blueberry fields.
Despite the differences that distinguish N. arbuti, N. australe and N.parvum, these species showed a similar growth pattern in response to temperature and showed very similar responses to changes in the aw of the medium. Corroborating a previous study (Espinosa et al., 2009), these Neofusicoc-cum species showed optimal growth at 25 °C and N. parvum exhibited the fastest mycelial growth. Similar studies have reported 25 to 35 °C as the optimum temperature for the germination of the conidia of other species of Botryosphaeriaceae (Pennycook and Samuels, 1985; Sutton and Arauz, 1991; Úrbez-Torres et al, 2010; van Niekerk et al., 2004; Wright and Harmon, 2010). It is of interest to note that the mycelial growth of B. dothidea obtained in Georgia, USA, was optimum at 30 °C (Kohn and Hendrix, 1983) and that conidia of N. parvum from grapevines in California germinated abundantly at 35 °C, although very little at 40 °C (Úrbez-Torres et al., 2010). The differences in optimum temperature obtained with isolates of Neofusicoccum from blueberries in the present work, as compared to B. dothidea and other Botryosphaeriaceae, may be attributed to natural variation between species of the Bot-ryosphaeriaceae family and the adaptability of Neofusicoccum spp. to environmental conditions.
In conclusion, the results of this study suggest that the aw, temperature and the interaction of these factors affect the development of the stem canker of blueberry caused by species of Neofusicoccum. Nevertheless, before a conclusion is established it will be necessary to validate this information under field conditions.
The authors are grateful for financing provided by the Comisión Nacional de Investigación Científica y Tecnológica (Conicyt), Fondecyt Project 1100246, in carrying out this work.
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Received January 4, 2011. Accepted June 20, 2011.
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