Differential leaf gas exchange performance of mango cultivars infected by different isolates of Ceratocystis fimbriata

150 Scientia Agricola http://dx.doi.org/10.1590/0103-9016-2015-0022 Differential leaf gas exchange performance of mango cultivars infected by different isolates of Ceratocystis fimbriata Wilka Messner da Silva Bispo, Leonardo Araujo, Wiler Ribas Moreira, Leandro de Castro Silva, Fabrício Ávila Rodrigues* Federal University of Viçosa – Dept. of Plant Pathology – ABSTRACT: Caused by the vascular fungus Ceratocystis fimbriata, mango wilt is considered to Lab. of Host-Parasite Interaction, Campus Universitário s/n be one of the most serious threats in mango-producing regions worldwide. However, changes − 36570-900 − Viçosa, MG − Brazil. in leaf gas exchange level and the mechanisms underlying host responses to this fungal infec- *Corresponding author <> tion remain poorly described. This study aimed to evaluate potential changes in the leaf gas exchange of different mango cultivars (Ubá, Espada, Haden and Tommy Atkins) in response to Edited by: Claudio Marcelo Gonçalves de Oliveira two Brazilian isolates of C. fimbriata (CEBS15 and MSAK16) to non-invasively assess cultivar variability in relation to the basal level of resistance to mango wilt. Both isolates, regardless of the cultivar, caused reductions in stomatal conductance and, thus, a reduction in CO2 assimilation via diffusive limitations. Taking into account the full length of the internal lesion and the radial colonization of the stem tissues, both isolates showed equivalent aggressiveness when inoculated into the Haden and Tommy Atkins cultivars. Conversely, when compared to the CEBS15 isolate of C. fimbriata, the MSAK16 isolate was more aggressive in cv. Espada and less Received January 26, 2015 aggressive in cv. Ubá. Accepted July 21, 2015 Keywords: Mangifera indica, mango wilt, photosynthesis, vascular disease Introduction Caused by the vascular fungus Ceratocystis fimbriata (Halsted, 1890), mango wilt is considered to be one of the most serious threats in mango-producing regions worldwide (Masood and Saeed, 2012), often leading to tree death and the decline of entire orchards (Ribeiro, 1997; Masood and Saeed, 2012). During infections by vascular pathogens, including C. fimbriata, vascular disorders usually lead to a decrease in hydraulic conductivity and the ensuing leaf water deprivation substantially affects plant growth and metabolism as a whole, reducing leaf gas exchange and inducing large alterations in source-sink relationships (Flexas et al., 2004, 2007; Roitsch, 1999). Crop yield has a high dependence on biomass production (Johnson, 1987), since the amount of carbohydrate supplied to fruits during their formation depends directly on the amount of assimilates produced during the photosynthetic process (Fishman and Génard, 1998; Rosati et al., 1999; Le Roux et al., 2001). Thus, because photosynthesis, and ultimately the source-sink regulation of sugar partitioning, is related to (and depends on) a healthy leaf area and the integrity of the plant, it may be assumed that, as a consequence of the alterations in leaf gas exchange caused by disease, considerable reductions in crop yield result from pathogen invasion and establishment in plant tissues. Photosynthesis may be considered a component of the integrated plant system, and thus, it is one of the physiological processes that is most sensitive to several abiotic and biotic stresses (Berry and Bjorkman, 1980; Baastians, 1991; Elings et al., 1999; Aucique-Perez et al., 2014). Therefore, the assessment of changes in leaf gas exchange may assist in elucidating the high variability Sci. Agric. v.73, n.2, p.150-158, March/April 2016 between mango cultivars in terms of their basal level of resistance to mango wilt. A better understanding of the relationship between C. fimbriata isolates and mango cultivars will be very important for plant breeding programs whose goal is screening accessions for resistance to the disease. Hence, considering the substantial yield losses in producing areas and the consequent need for more information about its effect over mango physiology, the main goal of this study was to assess the contrasting behavior of a number of mango cultivars against infection by certain C. fimbriata isolates in terms of plant leaf gas exchange performance. Materials and Methods Plant material Approximately one-year old mango plants, cultivars Espada, Haden, Ubá, and Tommy Atkins, were obtained from a commercial orchard in Dona Euzébia municipality, in the state of Minas Gerais, Brazil (-21o18’59” S, -42o48’38” W, and 222 m). All cultivars were grafted onto plants from the cultivar Imbú, which is widely used as rootstock in the Zona da Mata region, Minas Gerais, Brazil. Saplings were transplanted into plastic pots containing 8 kg of substrate consisting of a mixture of soil, sand and manure in a ratio of 2:1:1. The plants were maintained in a greenhouse (temperature of 30 ± 2 °C and relative humidity of 70 ± 5 %) for two months before the beginning of the experiments. The plants were irrigated and fertilized as needed. Inoculation procedure The procedure was largely performed as previously described (Bispo et al., 2015). The CEBS15 and MSAK16 isolates of C. fimbriata were used to inoculate 151 Bispo et al. the plants. These isolates were obtained from symptomatic mango plants collected, respectively, from the following municipalities in Brazil: Brejo Santo, in the state of Ceará (07°29’34” S, 38°59’06” W, and 381 m), and Aquidauana, in the state of Mato Grosso do Sul (20°28’15” S, 55°47’13” W, 147 m). The isolates were preserved according to Castellani's method (Dhingra and Sinclair, 1995). Plugs of malt extract-agar medium containing fungal mycelia were transferred to Petri dishes containing potato dextrose agar (PDA). After three days, the PDA plugs containing fungal mycelia were transferred to new Petri dishes containing the same culture medium and maintained in an incubator chamber (temperature of 25 °C with a 12-h photoperiod) for 14 days. The plants were inoculated according to Al-Sadi et al. (2010) with a few modifications. Stem disks (10-mm diameter and approximately 2-mm wide) were removed from the stems using a punch at approximately 5 cm above the graft scar. A PDA plug (10-mm diameter) obtained from a 14-day-old fungal colony was carefully placed in the punch hole. Each hole containing a PDA plug with fungal mycelia was carefully covered with a piece of moistened cotton and then wrapped with parafilm to maintain adequate moisture for fungal infection. The holes on the stems of the plants receiving only PDA medium plugs served as the control treatment. Disease assessments Disease development was evaluated at 28 days after inoculation (dai). The upward, downward and radial colonization of the stem tissues by fungal hyphae were determined by measuring the distance (length in cm) from the inoculation site to the edge of the internal necrotic tissue using a digital caliper. The upward relative lesion length (URLL) and th (...truncated)