Fruit tree diseasesThis article describes in more detail the damage caused by "black bark blight" – also known as Diplodia bark blight. It is also about the harmful agent Diplodia mutila, which was investigated in more detail in the research project of the Geisenheim Research Institute. 1 2
Since the hot and dry year of 2003, a new pest, and thus a bark disease initially classified as novel, has given rise to concern in orchard stands, as it has led to the death of young apple trees. In the meantime, black bark blight has also spread to organic orchards 3 and has also been found on pear trees there. 4
Black bark blight is favored by heat, drought and nutrient deficiency. Experts therefore expect the situation to worsen in the course of the climate change. 5
The black bark blight is a fungal disease. Black discoloration on the previous year's cuttings is a first indication that there is a spreading source of the highly contagious harmful fungus in the vicinity of the tree.
Treatment by cutting out infested areas is only possible at an early stage of infestation (see section E). However, preventive measures are important, such as z.B. a year-round white trunk coat and the best possible supply of water and nutrients, which will be discussed in more detail from section F onwards. The black bark blight is currently the largest. Very serious danger for our orchards.
B) BULLFIRE
The term bark blight covers various types of damage to fruit trees. The following overview 6 of bark damage on fruit trees and its cause of damage resp. its damaging agent (fungi or bacteria) indicates the classification of bark blight.
C) FUNGI FROM THE GENUS DIPLODIA
Black bark blight is a fungal disease caused by fungi from the genus Diplodia. The genus Diplodia is extraordinarily diverse, with more than 1000 species, and more than ten Diplodia species worldwide cause diseases of apple and pear. 7
The Diplodia fungus has been known as a weak parasite in Central Europe since the early 19. Diplodia fungus was described in the nineteenth century and has so far caused damage mainly in forests, predominantly on conifers. It now occurs worldwide, but previously with serious damage only in much warmer climates. 8 In Saxony-Anhalt, the Diplodia fungus was identified in dying oaks in 2019. The Diplodia fungus lives both on dead material and symptomlessly in plants. Is present everywhere due to this way of life. When weather conditions are favorable for it or. after damaging events, it can cause diseases by infecting wounds and young ties. Vital plants form defense zones. Thus limiting the spread of the fungus. However, after successful infection, the Diplodia fungus can survive for several years without symptoms, and after renewed stress events (z. B. drought) will then become aggressive after all. 10 At high temperatures – the optimum of Diplodia is between 25-30°C – the harmful fungus develops particularly quickly. 11
Black bark blight has also been known for a long time in orchards. It occurred sporadically until 2003. Occurring mainly in old apple trees. (Fig. 1)
From 2003 onwards, a new type of damage suddenly appeared (see Fig. 2) on young apple trees, which led to their death.
In the observation it was noticeable that on the meadow orchards the new damage pattern appeared especially when
If there was firewood stored in the vicinity or old trees affected by black bark blight and if the young apple trees were not cared for (whitewashing, watering, etc.), the fungus would not have spread.) had taken place.
The extent of the damage and the lack of explanation of the causes led to a research project at the Geisenheim Research Institute in 2008. Thereby the Diplodia mutila among the various pathogens detected (u.a. Diplodia mutila, Diplodia seriata and Diplodia malorum) as particularly aggressive. 12 As a debilitating parasite, it occurs especially during drought stress. Oliver Martinez showed in his research that the water supply has the strongest influence on the course of infection of the Diplodia mutila had. 13 14
While black bark blight was initially only found on apple trees in orchard stands, the extremely potent pathogen was also detected on pear trees in 2019. It affects not only shoots (trunk and branches), but also leaves and fruits, and the infestation depends on the variety. 15
The Diplodia pest generally benefits from mild winters, warm and humid spring weather, and subsequent dry summers. 16
Diplodia is a mold fungus. 17 According to the information available to us so far, it is not a cause of allergies. 18
How high the risk of infection of heavily infested trees for other trees is, has not yet been clearly clarified scientifically. To alleviate the infestation prere in a plant, clearing seems to be an appropriate control method. 19
In the observation, the prere of infestation is very high in orchards, as soon as there is already an infestation of black bark blight in the own or in the neighboring orchards. The risk of infection is particularly high in this case!
Old and cut wood must be removed and burned, since the Diplodia fungus prefers to develop its reproductive organs on dying and dead wood.20 Under no circumstances should it be moved to composting facilities.
D) DAMAGE DESCRIPTION:
Experts describe the damage as follows: 21 22 23
Bark infections can initially be asymptomatic and only develop during periods of weakness of the tree (drought, high temperatures, nutrient deficiency, mistletoe infestation 24 etc.).) can lead to an outbreak.
In the following damage pattern, the onset of black bark blight with the escape of a liquid can be seen (see Fig. 3)
An important characteristic is the formation of shallow, mostly slightly sunken bark necroses – dead tie parts – of the outer bark cambium, which are sharply delimited from the healthy tie (see Fig. 4 and 5). Mainly the trunk is affected. Strong branches also affected. The damage usually occurs on the sunny side of the trunk. 25
Black bark blight begins with a dark brown to black discoloration, under which the bark sinks in slightly. Often there are bark cracks or injuries nearby – see frost cracks on an apple tree in Fig. 6.
As the process continues, warts develop on the bark, eventually rupturing and revealing black round fruiting bodies of the fungus. This leads to pronounced blackening of affected bark areas, as shown in the following damage patterns (Fig. 7 and Fig. 
can be seen:
The black discolored wood sometimes has a cube-like structure reminiscent of wood burned by open fire (see also Fig.1 on page 3)
In the stem cross-section, a so-called black rot is visible in these cases (Fig. 9), which is caused by the cellulose degradation of the fungus in the colonized wood.
In some cases, the bark peels off completely (see also Fig. 10, Fig. 11 and Fig. 12).
Particularly affected are young trees, which usually still have a smooth and therefore particularly sensitive bark, so that large areas of the sapwood are exposed and are poorly covered or not covered at all.
Whether wound healing by overgrowth takes place depends decisively on the vitality of the tree and the size of the cut.
In the case of large-scale infestation or. in an advanced stage, further colonization takes place by secondary pathogens such as the split-leaf fungus, a wood-destroying white rot fungus, which additionally weakens the tree. 26
If the bark blight spreads to the entire trunk, which is the case especially with younger trees, the trees die.
As mentioned above, the Diplodia pathogen has also been identified on pear trees. For the damage pattern see Fig. 15.
In addition to bark blight symptoms, the Diplodia fungus causes leaf spots (Fig. 16) and fruit rot (Fig. 17) to mummification of the fruit.
Infection obviously requires injury in shoots, while leaves and fruits are probably also infected via natural openings (stomata).
This requires warm temperatures (> 10 °C) and periods of precipitation are required. At high temperatures (optimum between 25-30 °C) the fungus develops rapidly.
E) TREATMENT BY CUTTING OUT (early stage of infestation)
If infestation by the harmful fungus Diplodia is detected at an early stage, cutting out the infested area can lead to lasting success.
The following photo shows a rehabilitation successfully carried out 3 years ago by cutting out the Diplodia fungus infestation in the trunk of a young tree. The tree has closed the wound well.
Even small infestations that have occurred at the interfaces after a planting pruning/pruning can still be rehabilitated by cutting them out.
It is important to take special care when cutting out the damaged area:
F) PREVENTIVE MEASURES
Good tree care and good water and nutrient supply are considered the most crucial preventive measures. The white color reduces temperature differences due to solar radiation in winter. Prevent the bark from heating up in summer. 27 It is recommended to provide a white lime coating all year round. Ready-to-use coatings are now available that adhere for several years. 28 Maintain this white coating well into the leading branches in subsequent years, as long as coating is possible, d.h. until the bark has formed.
– Deep watering during dry periods: with approx. 60 liters of water per young tree resp. 100 liters for older trees, especially important also in the months of March, April and May, preferably by drip irrigation (water standing too long in the tree disc will otherwise cause another disease, collar rot), buckets or mason jars with holes drilled in the bottom are suitable for this purpose.
– Open tree discs: to avoid water competition around the tree
– Trunk protection measures: against game browsing and mowing damage, check tethering material regularly, trunk painting (s.o.)
– Fertilization: regular fertilization with potassium (not only in spring, but also in autumn), d.h. not too rich in nitrogen: potassium is considered an important prerequisite for the formation of the trees' natural frost hardiness
– Soil improvement measures: Mulch layer (but not from grass cuttings, underneath the temperature increases), root strengthening mycorrhizal fungi. 29 Sandy soils heat up more due to solar radiation than humus-rich soils – compost contributes to soil improvement.
– Cleaned and disinfected tools: to limit the spread of disease in general, the following advice applies: only work with two shears/saws each in a meadow, d.h. to prevent the transmission of diseases at least from tree to tree, place a pair of scissors/saw in a bucket of 80% alcohol immediately after pruning in the first tree and then continue working in the next tree with the second pair of scissors/saw thus disinfected. 30
– Control and eradication of voles: 31 root wire protection for new planting 32 , installation of perches for birds of prey
– Old and cut wood: absolutely remove and destroy, no wood chips as a mulch layer
– Site selection when planting new orchards: Dry, south-exposed sites and sites far from groundwater are critical. 33
– Variety selection: Variety selection appears to be an important influencing factor with respect to resistance to black bark blight – see section J) under "Current Research.".
G) CONCLUSION ON THE MANAGEMENT OF BLACK BARK BLIGHT:
Before first pruning: scan own stand for black bark blight infestation. Also check the neighboring properties to see if there is any unrecognized danger lurking there.
Trees infested with black bark blight – with the exception of young trees (see "Treatment for cutting out" in Section E) – are not to be saved. In the case of clearing, the requirements of the responsible lower nature conservation authority must be observed.
Regarding the risk of further spread of black bark fire and pruning work: any Interface in an environment affected by black bark blight is already a potential entry point for the harmful fungus. If each new interface does not IMMEDIATELY immediately after the cut is sealed with a fungicide-containing miracle sealant[i], then the fungus has also arrived in the new interface. Any untreated cut will be black in color the following year at the latest – these black cuts are foci of infection. 35
As a further precaution: in your own stand, make as few cuts as possible and invest the time thus saved in protective measures – see "Preventive Measures" in Section F.
In a region severely affected by black bark blight: for now, more reluctance to plant new apples or pears – those who plant anyway must (be able to) take more care than ever of preventive tree care.
H) OUR ORCHARDS ARE IN GREAT DANGER
Black bark blight is probably the biggest threat to our orchards. Climate change in general has greatly increased the incidence of fungal and viral diseases, and mistletoe infestation has also become rampant. In addition, there are many other winners from climate change, such as invasive scale insects .