Reaction of tangerines genotypes to Elsinoe fawcettii under natural infection conditions

A citrus scab disease, caused by Elsinoe fawcettii, is currently found in all citrus areas throughout Brazil. That being, given the importance of this casual agent, the behavior of tangerines and hybrids influenced by this pathogen was evaluated under natural infection conditions. This study was performed with plants around 15 years old without irrigation; 100 fruits of three plants were collected during harvest season, using a grade scale varying from 0 (absence of symptoms) to 6 (severe symptoms) the level of disease severity was determined. Among the cultivars, citrus scab resistance was observed in Citrus deliciosa, C. tangerina, C. nobilis; a mandarin hybrid (C. nobilis x C. deliciosa) and a satsuma hybrid (C. unshiu x C. sinensis). Among the other genotypes, symptoms were observed with levels of severity ranging from 1 to 3, indicating moderate resistance.


INTRODUCTION
In many citrus production areas around the world, Elsinoe fawcettii is one of the main fungi diseases found.It attacks a wide variety of citrus species and cultivars, resulting in scab disease on leaves, twigs, and fruits (Timmer et al. 1996, Hyun et al. 2007, Goes and Reis 2009).
In Brazil, E. fawcettii is responsible for citrus scab.The disease is widespread in many humid, citrus-cultivating areas around the world and decreases fruit values on the fresh-fruit market (Feichtenberger et al. 1986).In young plants or under severe infection, it may cause significant fruit drop.Furthermore, roughness in the fruit peel caused by this disease reflects the difficulty to control some important citrus plagues like citrus leprosis mites (Brevipalpus phoenicis) (Rodrigues et al. 2003).
The necrotic lesions caused by E. fawcettii appear as a slight blemish and often form erumpent scab pustules comprised of fungal hyphae and host tissue.The affected leaves may develop lesions with warty or protuberant pustules emerging from one side of the leaf and a corresponding depressed area on the opposite side (Wang et al. 2009).

MC Souza et al.
Currently fungicides, mainly cuprics, represent the primary means of controlling citrus scab (Feichtenberger et al. 1986, Mondal et al. 2007).Another alternative is to grow more resistant varieties, standing out as being the cheapest and easiest to use (Camargo and Bergamin Filho 1996).
Among the studies performed to select resistant citric varieties, Faling et al. (2004) presented the Kiyomi tangor hybrid from mandarin and orange as resistant to citrus canker and scab.AiJum (2004) presented the 'Cuipijingan' (Fortunella japonica) cultivar as resistant to citrus canker, scab, and tolerant to the Citrus tristeza virus.Yoshida et al. (2003) presented the 'Puchimaru' (Fortunella sp.) cultivar, a cross between 'Naga Kinkan' and 'Ninpou Kinkan' tetraploid, as being resistant to citrus canker and moderately resistant to scab.
This study sought to select potential agronomic genotypes of tangerine resistant to E. fawcettii, under natural infection conditions.

Selection of the tested materials
Botanical classification of the varieties and hybrids was performed according to descriptions reported by Ortiz Marcide (1986).All of the elected citrus material (22 genotypes) belonged to the Germplasm Bank at the Estação Experimental de Citricultura de Bebedouro (EECB), located in the Northern region of the State of São Paulo (lat 20° 532 163 S, long 48° 282 113 W, and alt 680 m asl), Brazil, in a 6 m × 3 m spacing (555 trees per ha), on a total area of 1,944 m 2 .The climate is Köeppen's Cwa, with a mean temperature of 23.5 °C and an annual rainfall of 1,522 mm.The soil is classified as Haplustox (38 % clay) (Embrapa 1999).Given the disease history over the last years, it has become unnecessary to achieve pathogen inoculation in the experimental area.However, to ensure an appropriate inoculum level, fungicides were not sprayed in the critical phases of leaf susceptibility nor two years before the implementation of the experiment in fruits.Other cultural practices were performed according to the adopted methods used in commercial orchards.
One year prior to assessing the performance of the genotypes to E. fawcettii, the degree of potential agronomic value was evaluated.This latter measure was determined by previous evaluation of palatability and phenotypic characteristics such as appearance, coloration and ease of peeling, as well as resistance tests to Alternaria alternata.The genotypes evaluated are on Table 1.

Evaluation of disease severity
From April to May, 2008, when fruit maturation begins, 100 fruits were collected per genotype in three plants, totaling 300 fruits per genotype located in the middle of the plant as well as distributed across all four quarters.

RESULTS AND DISCUSSION
Among the resistant genotypes, 5 mandarins were found corresponding to one C. reticulata, one C. deliciosa, two C. tangerina, one C. nobilis; one mandarin (hybrid) resulting from crossing C. nobilis x C. deliciosa and one satsuma (hybrid) resulting from crossing C. unshiu x C.odeliciosa (Table 1).For the other genotypes, the infection level ranged from 1 to 3. Due to this, they were considered moderately resistant, not exhibiting any highly susceptible cultivar when compared to rangpur lime (susceptibility pattern).
In the present study, several degrees of disease severity were observed, even in fruits of the same species, as in clementines (C.clementina), mandarins (C.reticulata) and moderate from satsumas (hybrids) (C.unshiu x C. sinensis).However, in the case of species whose fruits were classified as moderately resistant, fruits with no symptoms were not observed, showing a good inoculum distribution among the plants.Variations in severity levels were also observed by other researchers in studies involving this pathosystem Citrus-E.fawcettii (Inque et al. 1985).In a similar study, Reddy et al. (1986), aiming to select resistant Rough lemon to E. fawcettii, also observed reactions varying from immune to highly susceptible.Among the selected immune fruits were 'Shomyndong', 'Milan', 'Khattazamir', 'Brazilian Rough lemon' and 'Chase Rough lemon'.
Researches aiming to select resistant tangerine species to E. fawcettii have been conducted worldwide.In this context, Matsumoto et al. (2003a) observed that 'Nishinokaori' tangor, a crossing of 'Kiyomi' tangor and 'Trovita' orange, was resistant to this pathogen.In the present study, except for Burgess SRA 412 mandarin, all other genotypes belonging to or deriving from crosses of C. reticulata showed moderate to low resistance to the fungus.
The resistance verified in Burgess SRA 412 mandarin, obtained from just one cultivar of C. reticulata, deserves more detailed researches.Bitancourt (1935) for example, observed that other tangerines of common ancestors, like 'Cravo' mandarin, showed to be susceptible to E. fawcettii and E. australis, differing from the results obtained.
An Encore SRA 190 (C.nobilis x C. deliciosa) mandarin hybrid showed to be resistant to E. fawcettii, like 'Setoka' (C.sinensis x C. reticulata) cultivar, a crossing of Kuchinotsu nº 37 (Kiyomi x Encore nº 2) and Murcott tangor, which is resistant to scab and also Xanthomonas axonopodis pv.citri (Hasse) (Matsumoto et al. 2003b).Thus, given the history, it appears that the resistance observed is due to Encore SRA (B) mandarin genes, showing its potential for studies in breeding programs aimed at controlling this pathogen.
Finally, Commune, Caffin, Bruno, Burgess, Peau Lisse, Zanzibar, Beauty of Glen Retreat, Rode King and Encore cultivars, have demonstrated good agronomic characteristics; being resistant to E. fawcettii and, according to Reis et al. (2007) and Souza et al. (2009), also resistant to A. alternata, presenting the viability of a promising economic exploration.