Notas Científicas Reaction of arracacha genotypes to the root soft rot caused by Pectobacterium chrysanthemi

The purpose of this paper was to screen thirty-two arracacha genotypes for their reaction to root soft rot. Twenty roots of each genotype were inoculated with two Pectobacterium chrysanthemi isolates in a randomized experiment (10 roots/isolate). After inoculation, roots were individually wrapped with PVC film and kept at 26oC in closed plastic bags. Soft rot lesions were recorded after 36 hours and genotypes were grouped in four classes of susceptibility by cluster analysis: 10 were less susceptible, 16 intermediate, 3 susceptible and 3 very susceptible. All the tested arracacha genotypes showed only variation in the degree of susceptibility.

Arracacha (Arracacia xanthorrhiza Bancroft) is a typical South American vegetable root crop from the Andean region, which was probably introduced in Brazil at the beginning of the 20 th century. Arracacha is grown and consumed mainly in the South-Southeast regions of Brazil. In 2001, production was estimated in 120,000 t and the planted area reached 13,000 ha. This vegetable is regarded as a rustic, non-demanding crop by most farmers, because few diseases and pests occur during the long 8-10 month cycle (Santos & Carmo, 1998;Santos et al., 2000;Henz, 2001).
During summer, the soft rot caused by Pectobacterium spp. in arracacha roots can cause losses of up to 100% in only three days (Henz, 2001). The disease is known as "mela", the Portuguese word that better describes the sticky soft rot of the roots. The three most important pectolytic bacteria, Pectobacterium carotovorum subsp. carotovorum (=Pcc), P. carotovorum subsp. atrosepticum (=Pca) and P. chrysanthemi (=Pchr), are involved in the disease in Brazil (Romeiro et al., 1988;Henz, 2001). In general, Pcc was considered as the most important subspecies causing soft rot in arracacha roots in Brazil (Lopes & Quezado-Soares, 1997), but Pchr was determined as the prevailing species in the summer epidemic outbreaks registered in São Paulo at the CEAGESP wholesale market, the main trade center of fruits and vegetables in the country (Henz, 2001).
There are few possibilities for controlling soft rot, considering the prevailing handling system used for washing, grading and packing arracacha roots in Brazil. Most of the arracacha commercialized in São Paulo is grown in the states of Paraná, Minas Gerais and Santa Catarina. After harvest, the product is transported at night by trucks in 32 kg plastic trays from distances ranging from 400 to 700 km to packing houses located in Piedade and Tapiraí counties, 150 km away of the wholesale market of São Paulo (Santos et al., 2000;Henz, 2001). Presently there is not any chemical product officially registered to control diseases or pests in arracacha at the Brazilian Ministério da Agricultura, Pecuária e Abastecimento. As soft rot progresses very quickly under conducive conditions, preventive measures seem to be the best strategy to reduce the disease incidence (Henz, 2001). The identification of less susceptible arracacha genotypes can be important altogether with other measures.
Breeding new arracacha cultivars is incipient in Brazil, and one of main constraints is the lack of genetic diversity available (Sediyama et al., 2000). Important agronomical traits for arracacha are presently poorly exploited by research. In the first attempts of breeding a new arracacha cultivar, yield, precocity and regional adaptation were the most important characteristics to be pursuited by plant breeders. Fortunately, the rootknot nematode (Meloidogyne spp.) and the soft rot caused by Pectobacterium are the few diseases presently considered as limitant to arracacha in Brazil (Henz, 2001). The narrow genetical base represented by the three cultivars grown in the country can be threatened by new or emerging pests and diseases. Cultivars 'Amarela Comum' and 'Branca' probably represent more than one genotype, as they have been cultivated in Brazil since the introduction of arracacha. Farmers usually maintain their own mother plants in the field or simply replant cormels from the harvested plants (Santos et al., 2000). Genotypes originated from botanical seeds of yellow-root plants collected in Brazilian arracacha farms showed a great genetic variability for many agronomical traits, such as crop cycle, yield, number, size and color of roots and root shelf life, and were considered highly heterozigous by Hermann (1997).
The reaction of vegetable crops to soft rot is closely related to the screening method, selecion of species/ subespecies of Pectobacterium, characterization of the aggressiveness of isolates, definition of inoculum concentration and incubation conditions. Apparently, the reaction of potato tubers to the three main pectolytic Pectobacterium was independent of the species/ subspecies chosen (Austin et al., 1988;Wolters & Collins, 1994), and the same was observed in carrot roots by Michalik et al. (1992). In these papers, isolates of Pcc or Pchr were proven to be more aggressive at 22-30 o C when compared to Pca. A similar approach was developed for the Pectobacterium × arracacha pathosystem, in which inoculation techniques, inoculum concentration, differences in isolate aggressiveness and methods of disease evaluation were studied (Henz, 2001).
The objective of this paper was to screen 32 arracacha genotypes from the 'Embrapa Hortaliças' germplasm collection for their reaction to Pectobacterium chrysanthemi, the main subspecies involved in root soft rot in Brazil.
The arracacha genotypes were grown under field conditions in Brasília, DF, Brazil, during a period of ten months following the technical recommendations for the region (Santos & Carmo, 1998;Santos et al., 2000). Each genotype was grown in a split plot experimental design, with ten plants/row and four replicates. After harvest, roots were hand washed and kept at laboratory room condition (22-25 o C and 55-75% RH) for four hours to dry superficially.
Two isolates of Pectobacterium chrysanthemi of high (B11) and intermediate (B12) aggressiveness (Henz, 2001) were previously selected for this trial. The Pchr isolates were cultivated in Petri dishes containing the medium of Kado & Heskett (Klement et al., 1990) and kept at 27 o C during 24 hours. Inocula were prepared by suspending bacterial cells in sterile water, and the inoculum concentration adjusted to 10 8 cfu/mL by spectrophotometer (wavelenght of 550 nm, 52% of transmitance) through a calibration curve previously obtained (Henz, 2001). Each isolate was inoculated in ten roots of each genotype at two different points, in a randomized design. The inoculation technique consisted of a wound made by a sharp metal needle (0.3 cm of diameter and 1.2 cm deep) at the distal and proximal ends, aproximately 5 cm from each other, and the deposition of 15 µL of inoculum. After 1 hour, roots were individually wrapped in PVC films, packed together in closed plastic bags and stored during 36 hours at 26 o C. Check treatments consisted of two roots of each genotype inoculated with sterilized water. Evaluation was made by measuring the average diameter of the soft rot lesion, and clones were grouped in classes of susceptibility based on the average diameter of soft rot lesion caused by the two Pchr isolates by the cluster analysis of SAS software (SAS Institute, 1985).
The soft rot lesion diameter after 36 hours of inoculation ranged from 1.7 to 3.4 cm; checks treated with sterilized water showed no symptoms (Table 1). All the 32 arracacha genotypes inoculated with the two isolates of Pchr were considered susceptible. The average diameter of soft rot lesions caused by the Pchr isolate B11 (high aggressiveness) was 8.7% larger when compared to those caused by the isolate B12, except for four arracacha genotypes (96003,96031,96386,96406). The soft rot diameter of the lesions caused by isolate B11 ranged from 1.97 cm in the clone 94257 (yellow roots) to 3.4 cm in the genotype Ecu1216 from Ecuador, a white root clone. Interaction between the reaction of the arracacha clones and the two Pchr isolates was not consistent and the two lesion diameters were used to perform a cluster analysis.
Arracacha genotypes were separated into four groups by cluster analysis, average Euclidean distance and single linkage method (SAS Institute, 1985), accordingly to the soft rot lesion diameter (Table 2). Ten clones were considered as less susceptible (soft rot lesion from 1.7 to 2.3 cm of diameter), 16 were intermediate (2.1 to 2.5 cm), three were susceptible (2.4 to 2.8 cm), and three were very susceptible (2.4 to 2.8 cm).
According to Pérombelon & Salmond (1995), there is no resistance or immunity to Pectobacterium spp. in cultivars of agricultural plants considered hosts of this group of pectolytic bacteria, only variation in the degree of susceptibility. In the potato × Pectobacterium pathossystem, attempts to categorize the reaction of cultivars not always produced consistent results, probably due to problems such as the different inoculation techniques used, maturity of tubers, contamination of tubers and other causes. Based on this, the results reporting the reaction of vegetable crops to Pectobacterium can be assumed as less susceptible or more tolerant to soft rot, although some clones obtained from crosses of Solanum tuberosum and other species or subspecies of the genus Solanum can really have high levels of resistance (French & De Lindo, 1979;Wolters & Collins, 1994). There are other papers reporting differences of susceptibility to soft rot, including potato cultivars (Wastie et al., 1988), lettuce, sweetpotato, peppers and Chinese cabbage.
Among the arracacha genotypes selected for this trial, two white-rooted genotypes were from Ecuador and the other 30 genotypes were S1 lines, originated from the yellow-root clone 90134, collected in 1985 at a farm in the "Núcleo Rural da Vargem Bonita", Brasília, DF, Brazil. The S1 lines showed variability for many botanical and agronomical traits, such as yield, number, size and colour of roots. The selection of arracacha genotypes with less susceptibility to Pchr can be an important complementary measure to reduce the potential of postharvest losses and minimize the summer epidemic outbreaks of soft rot.
The 32 arracacha clones reacted differently to the inoculation, nevertheless none of them was considered resistant to Pectobacterium chrysanthemi. Based on this, it is possible to select clones less susceptible to soft rot among the germplasm of arracacha.