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versión On-line ISSN 0718-1620
Cienc. Inv. Agr. v.36 n.3 Santiago dic. 2009
Cien. Inv. Agr. 36(3):433-442
Crown and root rot of highbush blueberry caused by Phytophthora cinnamomi and P. citrophthora and cultivar susceptibility
Pudrición del cuello y raíces en arándano alto causado por Phytophthora cinnamomi y P. citrophthora, y susceptibilidad de cultivares
Alejandra Larach, Ximena Besoain, and Eduardo Salgado
Facultad de Agronomía, Pontificia Universidad Católica de Valparaíso, Casilla 4-D, Quillota, Chile. Corresponding author: email@example.com
Chile is the largest producer and exporter of blueberries in the southern hemisphere. During the 2006-2007 growing season, blueberry production increased in Chile as a result of an expansión of the planted área, reaching around 10,762 ha in 2007. Fungal diseases are of great importance in blueberry production; among them, Phytophthora root rot is a disease known worldwide as a major cause of death for highbush blueberry plants in the USA. Symptoms of Phytophthora root rot have been detected over the last four years in Chile. Nevertheless, the causal agents have not been previously described in Chile. This study aims to determine the pathogenicity of Phytophthora spp. isolated from highbush blueberry and to study cultivar susceptibility. The pathogenicity tests show that the species P. cinnamomi and P. citrophthora were the cause of crown and root rot of highbush blueberry. Significant differences in plant height, shoot growth stem diameter, and fresh weights of roots and aerial parts between inoculated and non-inoculated plants were observed. Blueberry cv. 'Toro' was the only cultivar resistant to P. cinnamomi, while both 'Elliot' and 'Toro' were resistant to P. citrophthora. Regardless of the Phytophthora species, cv. 'Biloxi' was the most affected. This is the first report of Phytophthora root rot in Chile and, to our knowledge, the first report of P. citrophthora affecting blueberry plants worldwide.
Key words: Blueberry diseases, highbush blueberry, blueberry cultivars, Phytophthora, Vaccinium corymbosum.
Chile es el principal productor y exportador de arándanos del hemisferio sur. Durante 2006-2007, la producción de arándanos (Vaccinium spp.) aumentó en Chile, como resultado de una expansión de la superficie plantada, alcanzando 10.762 ha en 2007. Las enfermedades fungosas tienen gran importancia en esta especie frutal siendo la pudrición radical producida por Phytophthora una de las principales causas de muerte de plantas de arándano alto en EUA. Durante los últimos cuatro años, se han detectado arándanos con pudrición radical en huertos de la región de Valparaíso, Chile, aislándose en todas ellas Phytophthora spp. Este trabajo tuvo como objetivos determinar la patogenicidad en arándano alto de aislamientos de Phytophthora spp., y evaluar la susceptibilidad de cultivares de arándanos actualmente plantados en Chile. De este modo, se demostró la patogenicidad de aislamientos de P. cinnamomi y P citrophthora , los que causaron pudrición al cuello y raíces en arándano alto. Existió una diferencia significativa entre las plantas inoculadas y las plantas testigo, para las variables altura de planta, crecimiento de brotes, diámetro del tallo, peso fresco de raíces y de parte aérea. En relación con P. cinnamomi sólo el cultivar 'Toro' fue resistente, mientras que los cultivares 'Elliot' y 'Toro' mostraron resistencia aP. citrophthora, siendo cv. 'Biloxi' el cultivar más afectado por P. citrophthora. De acuerdo con nuestro conocimiento, este es el primer reporte de P. citrophthora como causante de pudrición al cuello y raíces en arándano a nivel mundial y el primer trabajo que describe la pudrición del cuello y de las raíces por Phytophthora en Chile.
Palabras claves: Arándanos, arándanos altos, cultivares de arándanos, enfermedades, Phytophthora, Vaccinium corymbosum.
Most of the countries producing highbush blue berry (Vaccinium corymbosum L.) are located in the northern hemisphere, and the USA is the main producer and consumer at world level. Blueberry production has increased in several countries in the southern hemisphere, of which Chile is the main blueberry-producing and -exporting country. Blueberries were introduced around 1979, and the planted blueberry - area reached 10'762 ha in 2007 (C0RF0' 1993; USDA, 2005; ODEPA, 2007; INE, 2008). In Chile the main cultivars are 'Elliot', 'Briggitta' ;Duke', 'Bluecorp', O'Neal' , 'Blue Ray' and 'Berkeley' (ODEPA, 2007).
Phytophthora root rot was observed in blueberry bushes more than 40 years ago (Royle and Hickman, 1963). Previously, Raniere (1961), when studying blueberry plants showing yellowish leaves and early defoliation, roots necrosis, and vascular discoloration on the crown and stems, associated Phytophthora sp. withthis syndrome. However, Royle and Hickman (1963) identified P. cinnamomi associated to root rot of highbush blueberry. Recently, the same pathogen was isolated from blueberries in North Carolina, and the obtained isolates of Phytophthora were pathogenic in all the highbush blueberry cultivars (Clayton and Haasis, 1964; Milholland and Galleta, 1967). Currently, only P. cinnamomi has been described as a very important pathogen causing root rot in highbush blueberry; it has also been reported in Italy (Tamietti, 2003; Brannen et al., 2007).
Symptoms include yellowing or reddening leaves, growth cessation, defoliation, marginal leaves necrosis, and death of smaller terminal leaves and canes. Likewise, the root system of an affected bush is small, dark, and necrotic (Royle and Hickman, 1963; Clayton and Haasis, 1964; Milholland and Galleta 1967; Draper et al., 1971; Milholland 1975; Sterne, 1982; Clark et al., 1986; De Silva et al., 1999; Smith, 2002, 2006, 2007; Bryla and Linderman, 2007).
During the past fouryears, highbush blueberry plants with lower growth, foliar chlorosis, and reddish foliage have been detected and associated with crown and root rot. Phytophthora spp. colonies have been consistently recovered from diseased plants. Therefore, the objective of this study is to isolate and identify the causal agent of crown and root rot of blueberry and to evalúate the susceptibility of the main highbush blueberry cultivars currently produced in Chile.
Materials and methods
Isolates of Phytophthora were obtained from roots of highbush blueberry 'JeweF, 'Misty', 'O'NeaF, and 'Star' collected in commercial orchards located in Hijuelas and Villa Alemana (Valparaíso Región), Chile. The diseased plants were characterized by dieback, leaf chlorosis, foliage reddening, rootlets rot, and necrotic cortical lesions in the crown. The isolations were performed from roots and crowns washed thoroughly with abundant tap water, after which small pieces (1 cm long) of root and crown tissues from the edges of the necrotic lesions was extracted. Each sample was washed in sterile distilled water (SDW), dried on a sterile paper towel, and left aseptically for 40 min next to a burner. Each sample was plated immediately in MSP agar medium containing 18 g corn meal agar per liter, 10 mg pimaricine, and 100 mL pentachloronitrobenzene (PCNB) (modified medium P10PV, Tsao and Ocana, 1969). Plates were incubated at 24°C for 5 days in darkness. Hyphal tips obtained from colonies tentatively identified as Phytophthora spp. were sub-cultivated in MSP. Isolates were maintained in 10 mL SDW intubes at 15-20°C.
Additionally, four Phytophthora isolates obtained from diseased blueberries in October 2005 were included in this study.
Characterization and identification
Sporangia were obtained from mycelium pieces taken from actively growing colonies that were incubated in sterile carrot juice (500 g carrots boiled for 15 min 1 L SDW) on glass plates under continuous light for 48 h at 24°C. Then, the mycelial colonies were washed with SDW and treated for 3 min at 5°C with saline solution containing 2.36 g CaNO3, 0.5 g KNO3, and 1.0 g MgSO4 per liter, as well as 1 mL of chelated iron solution (13.0 g·L-1 EDTA, 7.5 g·L-1 KOH, and 24.5 g·L-1 FeSO4). Subsequently, the mycelium was washed with SDW, after which it was incubated in soil extract at 1% w/v for 48 h at 24°C under light. The presence of sporangia was determined under a light microscope. The shape, width, length, length/width ratio, presence or absence of papillae, number of pa-pillae, internal and external proliferations, and caducity or persistence of 30 sporangia per each isolate were determined.
Gametangia were studied in triplicate in single cultures or by pairing each isolate with known sexual compatibility types Al or A2 of P. cinnamomi in an MSP. Plates were incubated for 30 days at 21°C in darkness. The presence of oogonia and antheridia was determined under a light microscope after 5 days, and the formation of oospores was determined after 30 days of incubation.
The colony morphology and color was determined after incubation for 6 days at 24°C on potato dextrose agar acidulated with 1 mLL-1 1 N lactic acid (APDA). The effect of temperature on the mycelial growth was studied by observing the colony diameter after incubating in MSP at 5, 10, 20, 25, 30, and 35°C for 5 days. Three replicates were used for each temperature.
The identification of Phytophthora species was based on the taxonomic key s and on the description of each Phytophthora species (Stamps et al, 1990; Erwin and Ribeiro, 1996).
The pathogenicities of eight isolates (Par1-Par8) identified as Phytophthora spp. were studied in 18-month old blueberries. Highbush blueberries cv. 'Misty' were transplanted to 4-L pots with sterile soil substrate (1/3 organic soil and 2/3 pine leaves) and arranged on a 60-cm-high counter in a greenhouse with 24°C mean day temperature. In early winter (July), the soil substrate was infested with 100 mL per pot of a mycelial suspensión (105 propagules mL-1 ) obtained from APDA cultures. An equal number of control plants were treated with 100 mL of sterile water. Immediately after inoculation, as well as 30 and 60 days thereafter, all the plants were subjected to a 24-hour soil flooding.
The plants were fertilized weekly with a nutritive solution (nitrogen:phosphorus:potassium,1: 1:1). The soil substrate was maintained at pH 5.7 by adding 0.5 mLL-1 of phosphoric acid to an electrical conductivity of 0.6 dS m-1. The irrigation frequency was determined using a 15-cm tensiometer (Irrometer Company, California, USA) located in a control plant. Each plant was irrigated using 200 mL of water when the soil tension reached -0.04 MPa, restoring the soil humidity to field capacity (-0.033 MPa).
The presence of leaf chlorosis and reddening, root rot, and crown rot was recorded after 120 days of the inoculation. Plant height, sprout growth, stem diameter, fresh aerial weight, and root weight were also determined. To reisolate the pathogen from inoculated plants, small pieces of diseased tissues taken from roots and crowns were extracted from each experimental unit and plated in an MSP medium. The re-identification was made on the basis of the sporangia (Stamps et al, 1990; Erwin and Ribeiro, 1996).
Susceptibility of blueberries cultivars
The susceptibility of 12-month old blueberries 'Biloxi', 'Bluecrop', 'Brigitta', 'Duke', 'Elliott', 'Misty', 'O'NeaF, and 'Toro' to P. cinnamomi and P. citrophthora was studied. Plants were obtained from in vitro propagation and transplanted to 4-L pots containing sterile soil substrate (1/5 organic soil, 2/5 pine leaves, 2/5 sawdust). The test was performed between November and February in a greenhouse with an average day temperature between 20 and 30°C. Isolates Par-3 and Par-6, identified respectively as P. cinnamomi and P. citrophthora, were used. These Phytophthora isolates were the most virulent in pathogenicity tests previously performed. Each plant was inoculated with 100 mL of a mycelial suspensión (105 propagule mL-1 ) obtained from isolates on APDA. The control plants were treated with 100 mL of SDW. After the inoculation, all the plants were subjected immediately and after 30 days to 24 hof flooding.
The plants were fertilized as indicated before. Irrigations were performed only to restore the 10% soil humidity and maintained the soil at field capacity.
The symptoms were determined in inoculated plants, and the effect of the inoculations and re-identification of the pathogen were evaluated, as described above, 90 days after inoculation.
Design and statistical analysis
The treatments in the pathogenicity test and cultivar susceptibility were distributed according to a completely randomized design with four and three replicates of one plant each, respectively. The results were analyzed for variance, and means were compared according to Tukey's test (p = 0.05).
Isolation and identification
Based on the colony morphology and the morphology of the reproductive structures obtained, P. cinnamomi and P. citrophthora were identified (Table 1). The identification of the isolation Par-6 was corroborated molecularly by CABI Bioscience (IMI 396018).
The colonies developed by P. cinnamomi (isolates Par-1, Par-2, Par-3, Par-4, Par-5, and Par-8) were white in MSP. Mycelial growth was obtained between 20 and 30°C, but optimal temperature for mycelial was 25°C. A white cotton-like colony with a rosette aspect developed in APDA medium (Figure 1a). The mycelium was characterized by the presence of abundant coralloid to botryose hyphal swellings (Figure 1b) and the presence of terminal chlamydospores of 30 µm in diameter. Persistent, ellipsoid to ovoid non-papillate sporangia of 61.2 um x 37.0 um on average were obtained in a liquid medium. Sporangia had a length:width ratio of 1.7, and internal proliferations were observed (Figure 1c). Finally, the crossings made with P. cinnamomi reference strains (type Al and A2) evidenced the formation of smooth walls and hyaliñe oo-gonia, with an average diameter of 40.5 um, and oospores were produced in the presence of the compatibility type Al of P. cinnamomi. Therefore, a heterothallic compatibility and the compatibility type A2 were determined. The Antheridia were amphygenous (Figure 1d).
P. citrophthora (isolates Par-6 and Par-7) was characterized by the development of white and bare colonies in MSP. Mycelial growth was obtained between 20 and 30°C, but the optimal temperature for mycelial growth was at 25°C. In APDA, the colonies were white and cottony-like (Figure 2a). Hyphal swellings or presence of chlamydospores were not obtained. Papillate and deciduous sporangia were obtained in liquid medium, with a medium peduncle (higher than 20 µm and lower than 50 µm ). Sporangia were ovoid (Figure 2b), obpyriform, obturbinate, or distorted (Figure 2c-d), presenting one or two ápices, 50.1 um x 48.0 um in average, with a lengthwidth ratio of 1.8. No oospores were obtained in single cultures or in cultures that paired each isolate with Al or A2 of P. cinnamomi.
All the P. cinnamomi and P. citrophthora isolates caused cortical rot lesions on the crown and root rots. Foliar symptoms consisted of leaf chlorosis and partial reddening of the foliage. A dark brown discoloration of the xylem tissue was observed. The height, shoot growth, diameter of main stem, foliage fresh weight and root fresh weight were significantly lower in plants inoculated with either P. cinnamomi or P. citrophthora than in non-inoculated plants. For P. cinnamomi and P. citrophthora, isolations Par-3 and Par-6, respectively, were the most virulent isolates (Table 2). Isolates of P. citrophthora (Par-6 and Par-7) induced the same level of crown damage, while isolates of P. cinnamomi varied in symptom expression (Table 2).
P. cinnamomi and P. citrophthora were reisolated from inoculated plants. Non-inoculated plants yielded no Phytophthora after reisolation.
Regardless of the cultivar, the plants inoculated with P. cinnamomi and P. citrophthora showed vascular discoloration, cortical rot in roots, leaf chlorosis, and foliar reddening. Independently of the species of Phytophthora, blueberry cvs. 'Biloxi' and 'Duke' showed the most damage. Non-inoculated plants were without symptoms.
The blueberries 'Biloxi' and 'Elliot' inoculated with P. cinnamomi showed a significant decrease in plant height compared to non-inoculated plants. The blueberries 'Biloxi' and 'Bluecrop' showed a significant decrease in the diameter of main stem, cvs. 'Bluecrop', 'Briggita', and 'Duke' had a significantly decreased weight of fresh roots, and cv. 'Duke' showed a significant decrease in fresh aerial weight when compared to non-inoculated plants.
The blueberry cv. 'Biloxi'inoculated with/! citrophthora showed a significant decrease in the plant height, and the diameter of the main stems was significantly decreasedinthe cvs. 'Bluecrop' and 'Briggita'. The weight of fresh foliage was significantly decreased in cvs. 'Biloxi', 'Briggita', and 'Duke', and the fresh root weight decreased significantly inevs. 'Biloxi', and 'Duke' compared to non-inoculated plants (Table 3).
Phytophthora colonies were reisolated from diseased tissues of inoculated blueberry plants, after which P. cinnamomi or P. citrophthora were identified. There was no evidence of Phytophthora spp. in non-inoculated plants.
The morphological features of the Phytophthora isolates allowed the identification of P. cinnamomi and P. citrophthora in Chilean blueberries for the first time.
All isolates of P. cinnamomi obtained were heterothallic of compatibility type A2, which is in agreement with works previously reported (Clayton and Haasis, 1964; Sterne, 1982). The P. citrophthora isolations were sexually sterile, which is characteristic of most P. citrophthora isolates (Stamps etal, 1990; Erwin and Ribeiro, 1996).
All the isolates of P. cinnamomi and P. citrophthora were found to be pathogenic in highbush blueberry, and the reisolation from diseased tissues was successful, which allowed the fulfillment of Koch's postulates. However, differences in virulence were found among isolates. Similar differences have been reported in blueberries (Milholland, 1975) and other species (Zentmyer, 1980; Dudzinski et al, 1993; Robin and Desprez-Loustau, 1998; Linde et al., 1999). Among isolates of P. cinnamomi and P. citrophthora, Par-3 and Par-6 were the most virulent isolates. The isolates Par-3 and Par-6 showed, respectively, a decrease of 67.8 and 63.3% in stem growth, 39.1 and 32.1% in main stem diameter, 43.4 and 32.5% in plant height, 63.9 and 46.1% in fresh foliage weight, and 36.5 and 53.4% in fresh root weight, compared to non-inoculated control plants.
The symptoms developed by inoculated plants coincide with field symptoms commonly ob-served under field conditions in Chile, as well as with symptoms described for Phytophthora root rot elsewhere (Royle and Hickman, 1963; Clayton and Haasis, 1964; Milholland and Galleta, 1967; Milholland 1975; Erb et al, 1986; Tamietti, 2003; Smith, 2002, 2006, 2007; Bryla and Linderman, 2007). However, crown damage is uncommon and only reported in a few instances (Raniere, 1961; Milholland, 1995; Tamietti, 2003).
Based on the results obtained, blueberry cv. 'Toro' was seen to be highly resistant to P. cinnamomi, and the cvs. 'Elliot' and 'Toro' were highly resistant to P. citrophthora. However, additional evaluations using zoospores under field conditions are required before drawing a conclusión about the resistance to P. cinnamomi and P. citrophthora observed in blueberries.
To the best of our knowledge, only P. cinnamomi has been previously reported in highbush blueberry. Therefore, this work represents the first report on P. citrophthora as a pathogen in highbush blueberry on a global scale. Previously, P. citrophthora has been reported to affect citrus crops (Besoain et al., 1998; Vial et al., 2006), raspberry (Latorre and Muñoz, 1993), and kiwi (Latorre et al., 1991) in Chile.
The authors thank Sebastián Ochoa, Berries Patagonia, for providing the plant material used in this study, and Gladys Andrade and Iván Cortés for their technical assistance and help with conducting the pathogenicity tests.
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Received 06 January 2009. Accepted 27 April 2009