AL ROZACEELOR BEHAVIOR OF SOME APPLE VARIETIES GROWN UNDER SUPERINTENSIVE SYSTEM TO FIRE BLIGHT ( ERWINIA AMYLOVORA ) ATTACK

The fireblight of rosaceous species is a very important disease for fruit growing in general and for the pome fruits species cultivation as well, especially under the conditions of expansion of superintensive training systems and in the context of current climate changes. During 20162018, the reaction of some performant apple cultivars grown under superintensive system to the attack of fireblight (Erwinia amylovora Burill Winslow) was assessed and was dependent on the variety/rootstock combination, trees age, and environmental conditions, presence of the pathogen and its aggressiveness and management of some technological links. Our study revealed that the most sensitive cultivar/rootstock combinations were 'Dalinco/Pajam1', 'Topaz/M9', 'Topaz/T337', 'Dalinbel/T337' and 'Dalinette/M9' (11 years old). At the other end, the most resilient combinations were 'Golden Orange/T337', 'Ariane/M9', 'Iniţial/M9', 'Rebra/M9', i 'Rustic/M9' (11 years old) and 'Mitchgala/M9' and 'Golden Delicious/M8' (3 years old). In the future, to overcome their sensitivity to fireblight, the varieties 'Dalinco', 'Dalinbel', 'Dalinette' and 'Topaz' might be grafted on M7 or other tolerant low vigor rootstocks. Cuvinte cheie: măr, focul bacterian, agresivitate, soi portaltoi, vârstă, verigi tehnologice.


Apple crop. The economic importance
In temperate areas of the Globe, apple is the main tree fruit species due to the role of apples in human nutrition and due to certain agrobiological features such as: it is well adapted to the temperate climate, with the extreme values of the meteorological parameters and the low fertile soils, has the highest harvest index of all fruit species (Koslowsky et al., 1991), its fruit supports easy transportation at great distances and are a valuable source of raw material for the food industry.
In Romania the harvested area reached 55.528 ha and apple production was estimated at 467.259 t, which marketed was an important source of incomes.Under the conditions of expansion of superintensive training and growing systems and in the context of current climate changes, it is vital to know the behavior of new released or introduced apple varieties to pests and diseases attack, prior the establishment of the new high-density orchards, and to better manage the existent ones.
Fireblight spreading, host plants and main symptoms All over the World, the apple production sector can be affected by many pest and diseases which causes serious damages and economic looses.
Among them, fireblight is one oldest and most devastating.Under ideal conditions it can destroy an young apple or pear orchard or nursery in a single season.The pathogen overwinters in diseased shoots and surrounding bark plagues (Ivey L. M., 2016, Biggs A.R. and Steiner P.W., 2000) and during the vegetation period, the bacteria are spread on host plants bay insects, bees, strong winds, rains water and human intervention in the orchard.
On the host plants, the disease symptoms were noticed on all aerial parts, and under favorable condition progress in an exponential manner.
During the warm springs, bacterial ooze drops appear first on the flower stalks.The flowers which first hydrolyzed become brown, are fading and soon turn in black.If daily medium temperature is higher than 16 °C, the infected flowers are entirely colonized in few minutes (Longstroth M., 2013, Biggs A.R., Steiner P.W., 2000).The leaves are affected starting with the petiole and the mid vein, and then the infection spreads between leaves and along the growing shoots which takes the shape of 'J' letter and turn in brown (Ivey L. M., 2016).
In the wet periods, on the attacked shoots of sensitive cultivars, bacterial ooze droplets, having white, dark-yellow, orange or even red colors suddenly appears.On the shoots, the infections become visible within 167 days, with average temperatures higher than 13 °C.(Steiner P.W. 2000).
On the green fruits the disease symptoms appears after hails and thunderstorms and the heavy infected fruits becomes brown in less than two weeks.
During the vegetation period, the bacteria enter into the host using natural gates like wounds, broken stalks or shoot tips, stomata, lenticels.On the sensitive varieties, the infection move down to the phloem, to the trees limbs and trunks producing plagues and exudates and the death of entire trees.
Researches regarding fire blight and its confinement.
In order to restrain and control the disease spread and the consequent damages, intensive researches are carried out in many apple growing countries and in our Country as well in the fields of: assortment and rootstocks evaluation, pathogen biology and epidemiology, early detection and risk assessment, monitoring of the pathogen propagation into the ecosystems, technological methods aiming to reduce varieties susceptibility, use or adequate plant protection products, define the behavior and breeding of new tolerant or resistant varieties (Table 1).
The aim of this work was to evaluate some performant apple cultivars grown under superintensive system, to better know their behavior on the attack of fireblight (Erwinia amylovora Burill Winslow), to find correlation between some biochemical traits of shoots and fruits and the components of the attack and to define a set of measures to prevent and control the fireblight attack.

Material and methods
The researches were conducted during 2016-2018 at Research Institute for Fruit Growing Pitesti Romania on more than 40 apple varieties recently released or introduced in our Country.
Apple trees were grafted on six different low vigor vegetative rootstocks, grown under superintensive system with over 3000 trees /ha, trained as slender spindles and supplied with water and nutrients by fert-irrigation.
The experimental device was located on a plain terrain placed on the second terrace of the Arges River, on a low to medium fertile alluvial clay unit (more than 30% of clay; humus less than 1.7%; nitrogen index 0.33-1.43;PAL 1.3-2.5 mg/100g, but well supplied with potassium, up to 40 mg/100 g).Soil reaction is slightly acid (pH=5.8-6.8).
The orchard floor was covered with grass between the tree rows and cleared with total herbicides on stripes of 1.0-1.2m wide, along the tree rows.
During the dry periods the experimental plots received 6-7 l water/ m 2 /day.The data related to local microclimat were collected using the semi-automate weather station WatchDog (Spectrum Technologies Inc.), and was stored, processed and analyzed using the facilities of MS Office Excel 2010 and Specware Pro 9.0 module for early warning.
Using this module, the risk of infections with fireblight was assessed using the Cougar scale (where '0'=no risk; '1'=low infection risk; '2'=medium infection risk and '3'=high infection risk).
At 23 most promising apple varieties, during the vegetation period, observations regarding the disease attack frequency and intensity were done using a modified Van der Zwet scale, where 10=healthy tree and 0=dead tree).
At the same promising apple varieties, additional assessments were done on: growing shoots pH; green fruits pH and electric conductivity (mV) as well as dry mater content of the fruits at the harvest time.
After the evaluations, additional sanitation works we done to remove the damaged shoots.
The experimental data were stored, processed and analyzed, using MS Office Excel 2010 facilities.
Pictures were taken with a Samsung E90 digital camera and processed with the free software Irfan View 4.25 and Photofiltre 7.0.2.

Results on about the infection risk Erwinia amylovora on apple
The local microclimate data collected reveal that the years of the study (2016-2018) were very favorable for the fire blight attack on apples.
Assessment of the figure 1 shows that, in 2016 the most critical periods for possible infections were: end of April (Cougar risk=2.0),end of May (Cougar risk=2.0-2.5),June-July (Cougar risk=2.5=3.0) and the risk was maximum in August-September (Cougar risk=3.0).
Analysis of the figure 2 reveal that, in 2017 the most critical periods for possible infections were: May (Cougar risk=1.5-2)and June till September (Cougar risk=2.0-3.0).
Assessment of the figure 3 shows that, in 2018 the most critical periods for possible infections were: mid of April-mid of May (Cougar risk=2.0-2.5),mid of May till end of June (Cougar risk=3.0),July (Cougar risk=2.5-3.0) and August-September (Cougar risk=3.0).

Results on some biochemical parameters and behavior of perspective apple varieties to the fireblight attack
The results regarding some biochemical parameters of the shoots and fruits and behavior of some perspective apple varieties to the fire blight attack are presented in the It can be observed that, during the study period, the growing shoots pH ranged between 5.2 and 5.9 and the young fruits pH between 2.9-3.9.
As regard the young fruits cells conductivity, on the fruits of 11-12 years trees old, the indicator ranged between 190 mV at 'Rustic' variety and 240 mV at 'Golden Orange' variety.On the fruits of 3 years old trees, this indicator oscillated between 218 mV at 'Jonaprince' variety and 236 mV at 'Golden Delicious' and 'Golden Reinders' varieties (see also figure 4).
The assessments carried out during the study period revealed that, the apple young leaves and fruits are tolerant at the early infections with the Erwinia amylovora bacteria, as long as the cellular juice electric conductivity oscillate between 180-250 mV, and cellular juice pH oscillate between 2.5 and 4.5.
The fruits dry mater content at the harvest moment ranged between 9.11 and 14.66 °Brix on the fruits of 11-12 years trees and 12.0 and 13.8 °Brix on the fruits of 3 years trees.

Correlations between some biochemical traits of shoots and fruits and the fireblight attack components
The amount of accumulated data for the 23 perspective apple varieties offer us to the possibility to find some correlations between some biochemical traits of shoots and fruits and the components of the fireblight attack.
Assessment of the Figure 7 shows that the increase of the shoots cellular juice pH from 5.2 up to 6.0 and assimilates accumulation favored the intensity of the fireblight attack, the correlation found was r=0.8259.A strong correlation (r=0.6412) was also found between shoots cellular juice pH and the pathogen attack frequency (Figure 8).
Evaluation of the Figure 9 reveal a strong correlation between young fruits cellular juice pH and fruits dry mater content at the harvest time.(r=0.8420).
In the Figure 10 one can observe that fireblight attack frequency partially influenced the fruits dry mater content at the harvest time.The correlation value found was r=0.4538.DOI 10.33045/fgr.v34.2018.18http://publications.icdp.ro/index.php97

Results regarding the behavior of perspective apple varieties on the fireblight attack
The data and information accumulated during the study period allow us to classify the 23 perspective apple cultivars in five groups, according their behavior to the fireblight attack on lateral shoots (Table 3) and to define a set of technological measures aiming to prevent and control the fireblight attack in apple orchards trained under superintensive system and for better protect the new established ones.
Based on the accumulated information and data, we propose a set of technological measures aiming to prevent and control the fireblight attack in apple orchards trained under superintensive system and for better protection of the new established ones including: -Monitoring the plots with 'very susceptible', 'susceptible' or even medium 'tolerant' varieties as well as the proximity areas; -Use of the healthy planting material, 'tolerant' or 'resistant' to fire blight to establish new highdensity apple orchards.
-To stand with their sensitivity to fireblight, in the future, the varieties 'Dalinco', 'Dalinbel', 'Dalinette' and 'Topaz' might be grafted on M7 or other tolerant low vigor rootstocks.
-The orchards pruning might be done as much as possible during the dormant period of the apple trees.
-When orchard sanitation is needed, disinfection is mandatory for the tools used for pruning, using absolute ethylic alcohol or sodium hypochlorite, undiluted commercial product) and protection of the resulted wounds, using special wax.
-The infected wood resulted from sanitation pruning must be collected, removed and burned entirely.
-For the orchards established in growing areas with infection potential for very sensitive and sensitive apple cultivars, the fertilization and irrigation must be conducted very carefully; drip irrigation is preferred.
-In such apple growing locations it is very important to implement a preventive program of treatments designed to reduce the inoculum reserve and to control the damaging insects acting as fireblight vectors.This program should include: -Fungicides based on: copper, aluminium fosetil, mancozeb, acetometil-S-benzolar), etc.
-Biological products based on antagonists of the Erwinia amylovora bacteria.

Conclusions
During 2016-2018, the meteorological conditions were very favorable to infections produced by Erwinia amylovora on apples.
The apple varieties behave different on the fireblight infections according their genitors, the affected organ (flowers, bark, shoots, fruits) or the entire tree/rootstock combination, meteorological conditions and local microclimate, the growing system and applied technology, production level, etc.
Young apple leaves and fruits are tolerant to the early infections with the Erwinia amylovora bacteria, as long as the cellular juice electric conductivity oscillates between 180-250 mV and the cellular juice pH ranges between 2.5 and 4.5.
The cellular juice pH increase up to 6.0, and the assimilates accumulation, encourage the pathogen attack intensity.
Four correlations, statistically insured were determined: young shoots pH --fireblight attack intensity, r=0.8259; young shoots pH --fireblight attack frequency, r=0.6412; young fruits pH --soluble dry matter content of fruits at harvest time, r=0.8420; fireblight attack frequency --soluble dry mater content of the fruits at harvest time r=0.4538; The data and information accumulated during the study period allow us to classify 23 perspective apple cultivars into five groups, according their behavior to the fireblight attack on lateral shoots (Table 3) and to define a set of technological measures aiming to prevent and control the fireblight attack in apple orchards trained under superintensive system and for better protection of the new established plantings.