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An anthracnose canker on the branch of an apple tree associated with heavy burring and cankering on the rootstock trunk (Photo V. Krejzar) 

An anthracnose canker on the branch of an apple tree associated with heavy burring and cankering on the rootstock trunk (Photo V. Krejzar) 

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Burrknot is a swelling or rough knot that is com-posed of partially developed adventitious roots on trunks above the soil line, limbs or branches (Swingle 1925a, b, 1927; Rom & Brown 1979; Perry & Cummins 1990). Fusing of adjacent burr-knots and increase in burrknot size, occurring Supported by the Ministry of Agriculture of the Czech Republic (Pro...

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... of apple trees. Apple trees of cvs. Early Smith, Jonagold, and Gala on M.9 rootstock were planted (at spacing of 1.1 m within rows and 3.6 m between rows) at orchard Těšetice in the South Moravian Region in spring 2003. In Stošíkovice orchard, adjacent to Těšetice, trees of cv. Gala on M.9 rootstock were planted (at spacing the same as that at Těšetice) in autumn 2002. Planting material used at the two locations was obtained from the same commercial propagator and had been established from the same propagation stock materials. At Stošíkovice, one block of Gala trees in the orchard was planted on a gentle north-east-facing slope and another block on the adjoining flatland. After planting, similar management practices for intensive fruit production were applied annually in both orchards. Plastic trunk guards were used. The herbicide diquat (Reglone) was applied for root-sucker killing. Supplementary irrigation was supplied as needed using drip irrigation. Sixty trees of each of three cultivars were examined for burrknots in May or June and October 2007 and 2008. Parameters measured were: ( i ) incidence (%) of trees with symptoms of burrknot; ( ii ) burrknot severity (number of burrknots on above-ground portion of rootstock per tree; ( iii ) trunk diameter (cm); ( iv ) height of graft junction above the soil line; and ( v ) mean number of root-suckers per tree. The frequency of dead Jonagold trees and the proportion of necrotic bark tissue on above-ground portion of the rootstock trunk circumference of dead trees (i.e. 1/3, 1/2, 2/3 and 2/2, respectively) were recorded during 2007. The differences between various parameters were analysed using the analysis variance and Pearson correlation. The UNISTAT 5.0 package (UNISTAT Ltd., London, UK) was used for statistical analyses of the data. Erwinia amylovora Phytophthora spp. Because some symptoms observed on apple trees attacked by burrknot (i.e. cankers formed around burknots) were similar to those caused by Erwinia amylovora and Phytophthora spp., an examination for these pathogen was carried out according to methods of Schaad et al. (2001) and Tsao (1987), respectively. Attempts to isolate fire blight bacterium from necrotic rootstock trunk tissues were carried out using King’s B medium and nutrient agar. Apple fruits were used as bait material for isolation of Phytophthora spp. Stem associated pome fruit tree viruses can cause a decline in young plants. Therefore, the presence of Apple stem growing virus (ASGV) and Apple stem pitting virus (ASPV) was assessed in 32 trees of cvs. Gala, Early Smith, and Jonagold with various burrknot severity using one-step-RT-PCR (Kundu 2003a). Four samples were usually tested from a single tree (leaves and bark tissue both from rootstocks or suckers and apple scions). Tissues were ground in a mortar and pestled in liquid nitrogen. One hundred mg of ground tissue were used for total RNA extraction using the RNeasy Plant mini kit (Qiagen, Hilden, Germany) accord- ing to the manufacturer’s description. Primers used in this study are shown in Ta- ble 1. Pome fruit tree viruses were detected using one-step-RT-PCR kit (Qiagen, Hilden, Germany) (Kundu 2003a). The one-step-RT-PCR reaction contained 5 μl of the 5× one-step RT-PCR buffer, 1 μl dNTP mixture (10 mmol/1 dNTPs), 0.7 μl of the RT-PCR enzyme mixture, 1 μl Q solution, 10 pmoles forward and reverse primers and 2 μl of total RNA, and was carried out in microtubes. The reaction mixture was adjusted to 25 μl with RNase-free water. The reaction was carried out in a thermocycler (MJ Research, Waltham, USA) as follows: an RT step at 50°C for 30 min and an initial PCR activation step at 95°C for 15 min, then 31 cycles of 94°C for 30 s (denaturation), 55°C for 45 s (annealing), and 72°C for 1 min (extension). After the last cycle, a final extension step at 72°C for 10 min was added. The roughened, warty-looking circular or spheri- cal growths or swellings of the gall-like appearance occurred primarily on the rootstock portion of trunks, between the soil line and the graft junc- tion and predominantly near the graft union. The growths were formed by many partially developed adventitious roots as is typical for burrknot. The size of individual burrknots ranged from 5 mm to 25 mm in diameter (Figure 1) . In the area imme- diately above some burrknots, the trunk diameter was depressed (Figure 2) . Not infrequently, cankers were formed around burrknots. The canker tissue appeared as sunken, orange-to-brown areas in the bark (Figure 3). A crack eventually developed between diseased and healthy tissue. Some older cankers had a zonate appearance (Figure 4) (Figure 4) . When cankers encircled the greater part of the trunk circumference, tree vitality was clearly lowered as determined by the greater incidence of necrotic lesions, especially anthracnose cankers with typi- cal fiddlestring appearance (caused by the fungus Neofabraea malicorticis ) on the limbs and branches in the tree-top (Figure 5). No burrknots were noticed on the scion part of the trunk, limbs and branches of trees evaluated. A strong tendency to produce root-suckers was observed in many roots of trees surveyed (Figure 6). The graft junction was, on average, 19 cm (in the range from 7 cm to 30 cm) above the soil line. In most trees evaluated, the swelling around the graft junction was pronounced. Removal of bark above and below the place where scion and rootstock was joined revealed markedly thick and spongy bark tissue. In addition, symptoms that suggested the presence of some viral pathogens were observed, such as slight pitting and grooving in the wood on the rootstock part of the trunk (Figure 7) . Incidence of burrknots in cvs. Early Smith, Jonagold and Gala was 98%, 97%, and 92%, respectively. Burrknots were localised on the rootstock part of the trunk between the soil line and the graft junction. The mean severity of burrknots was the highest on cv. Jonagold (3.65), whereas on cvs. Early Smith and Gala severity was about 23% lower (Table 2). The number of burrknots per tree was not related to trunk diameter. . Symptoms of burrknot were exhibited on 73.3% of cv. Gala trees planted on the gentle slope and 70.0% of trees planted on a level site. Burrknot severity on trees planted on the gentle slope (1.85) was not significantly differ- ent than on the flatland (1.37) (Table 3). Burrknot incidence and the incidence of root- suckers were the highest in Jonagold trees (Table 2). However, correlations between the incidence of burrknot and incidence of root-suckers were not significant. Five years after planting, increasing dying of cv. Jonagold trees was recorded at the Těšetice orchard. Of 290 trees examined, 5.5% died. Each dead tree exhibited burrknots on the rootstock, associated with cankers which more or less girdled the trunk. Of the evaluated trees, 43% showed a complete girdling canker, 50% exhibited necroses encircling one-half to two-thirds of the trunk circumference, and 6% revealed necroses encircling at least one third of the trunk circumference (Table 4). (Table 4). No abnormal premature dying of young tress was observed at the Stošíkovice orchard. Atempts to isolate Erwinia amylovora, and Phy- tophthora spp. from necrotised tissues surround- ing burrknots on rootstock trunk tissues were not successful. Apple stem pitting virus (ASPV) and Apple s tem grooving virus (ASGV) were detected in all of 10 tested trees of cultivar Jonagold and in 10 trees of cultivar Early Smith. The incidence of ASPV and ASGV in 12 trees of cv. Gala was lower. The virus positive trees included individuals both with various burrknot severity and without symptoms of burrknots. From data shown in Tables 5 and 6 it is evident that there is no relationship between burrknot severity and the occurrence of ASPV and ASGV in the rootstock and scion parts of apple trees. Some burrknots infested with larvae of the red- belted clearing wing, Synanthedon myopaerformis, were noticed during surveying the orchard at Těšetice at the beginning of June 2008. After the reporting of an unusually high occur- rence of burrknots in commercial apple orchards in South Moravia, the question arises if the disorder of trees was initiated before or after their planting in the orchards. In agreement with statements from growers, no burrknot symptoms were noticed on trees before planting. Therefore, it is highly probable that de- velopment of burrknots had already been initiated in the nursery. Burrknots arise from root initials, which originate from root primordia. Most of- ten they are initiated at a node. A bud gap is the most common point of burrknot origin (Rom & Brown 1979). Development of root initials oc- curs consecutively as the stem elongates (Wolfe 1935). The scion/clonal rootstock apple trees are frequently developed under conditions that are conductive to the formation of preformed root initials, i.e., low light intensity, high humidity, and temperature in stool bed and propagation row, resulting from shading in these high planting density situations. The primordium may or may not continue development. It may lie dormant for months or even years and it usually takes several years for burrknot to develop visually apparent symptoms (Perry & Cummins 1990). When the trees are planted in the orchard, these preformed root initials may develop into burrknots on exposed portions of the clonal rootstock if low light intensity persists as a result of shallow plant- ing, weed growth, use of opaque trunk protectors or shading by low limbs. Furthermore, temperature and humidity is increased under these trunk pro- tectors (Rom & Brown 1979; Howitt 1993). Factors that might have contributed to an out- break of burrknots in the surveyed commercial apple orchards in South Moravia after tree plant- ing include shading (as a result of root-sucker and weed growth) and drip irrigation. It seems to be probable that plastic trunk guards, used both in Těšetice and ...

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... The occurrence of burrknots is peculiar to the genetic constitution of the cultivar, not being due to infection by biological agents (KUDELA et al., 2009). Rootstocks differ in the sensitivity degree to this disorder, being very marked in MM-111, MM-106, M-7 and M-26. ...
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