Bacteria of fruit species: sometimes underestimated biopathogens but of undeniable harmfulness

Among the different groups of bio-aggressors of our fruit species, bacteria are sometimes considered economically unimportant. It is true that, in comparison with insects and cryptogamic diseases, their number is much less, and with moderate temperatures our climatic conditions are less favorable to them than hot and humid climates.

This way of seeing things was challenged somewhat when, in the 1970s, all of Europe was infected with Rosaceae fire blight (Erwinia amylovora) which turned the lives of pear growers upside down. To begin with, they had to "take" a brutal shock, then learn to fight and live with this pest that was new to us.

Getting to know bacteria better

Bacteria are unicellular organisms of very small size (0.2 to 10 µm) that live isolated or associated in groups. They are characterized by the absence of a membrane around the nucleus. Their shape is very variable: spherical, stick, spiral…; most often with one or more flagella which ensure their mobility in a liquid medium. Pathogenic bacteria infect plants via a wound (rubbing of two stems, hail or frost damage, leaf scars, pruning wounds, etc.) or natural openings such as leaf stomata, lenticels and flower nectaries.

On the foliage, the main symptoms are discolored or translucent (= "oily") spots, or tissue rot. In the vascular system of plants, their proliferation prevents the circulation of the sap, with the consequences of a general withering of the overlying part, or the production of an exudate or gum. In other cases, the bacteria cause anarchic multiplication of the cells of the plant, which will form galls, tumors, numerous rootlets or burls.

Their dissemination can be ensured in various ways: rain, wind, contact with animals, humans (his clothes and tools), fragments of infected plants, etc. carry droplets or filaments carrying bacteria .

These are organisms that depend on warm and humid ambient conditions, and which then multiply very quickly. We quote multiplication rates of the order of 10,000 generations in 145 days!

There are more than 300 bacteria responsible for plant diseases. Their nomenclature is Latin: Family – Genus – Species – Variety or Pathovar (pv.) for pathogenic bacteria.

Pathogenic bacteria of fruit species

Pathogenic bacteria of fruit species in temperate regions are found in four genera: Agrobacterium, Erwinia, Pseudomonas and Xanthomonas.

Some of them infect several plant species, such as Agrobacterium tumefaciens or Erwinia amylovora. Others, on the other hand, have pathovars linked to a single fruit species: for example Xanthomonas arboricola (= campestris) pv. juglandis in walnuts, and pv. corylina in hazelnuts.

As we will see, there are few means of combating bacteriosis in our fruit species, and they are sometimes only partially effective. They are limited to preventive cultural measures such as prophylaxis – avoiding importing infected plants, not cultivating in plots suspected of having been infected before, removing and destroying infected plants or parts of plants – and rare phytosanitary interventions for which the number of usable chemicals is reduced to copper compounds and a few others. Generally speaking, excessive nitrogen supply to plants is a factor that increases plant susceptibility to bacterial infections.

Agrobacterium tumefaciens: crown gall

This bacterium, which lives in the soil, infects a large number of plant species: more than 640 species, including most fruit species, through wounds or lenticels. It is disseminated by rainwater, runoff or irrigation, by animals or human activity.

In general, it causes a weakening of the vigor of plants. It disrupts cell division, and thus causes tissue proliferation. These then take the form of irregularly shaped lignified tumors on the roots and stems. Its persistence in the soil is several years. Its incidence is greater in heavy and humid soils, or those rich in nitrogen.

There are several organic varieties (= biovars); biovars 1 and 2 are widespread on various plant species, while biovar 3 is mainly infective for vines.

Crown gall is common in nurseries, where it is introduced by planting infected rootstock or cuttings. Thereafter, a very long rotation is essential. When planting trees, it is advisable to dress the roots a few days before to allow the wounds to dry out. Removed tumors should be burned.

There is a control method that uses an antagonistic bacterium, Agrobacterium radiobacter strains K84 and K1026, which produces an antibiotic. It is used by soaking the plants before planting.

Agrobacterium rhizogenes: hairy gall

As its name suggests, this bacterium similar to the previous one causes the disordered formation of an abundant hair of roots on tumors of the roots or stems. It mainly affects apple, pear and plum trees.

Erwinia amylovora: Rosaceae fire blight or fireblight

This disease was described on the East Coast of the United States at the end of the 18th century, from where it spread to New Zealand in 1919, then England in 1957 on infected fruits from the Southern Hemisphere. It established itself in the pear orchards of Kent and in gardens in the London area, where ornamental plants of the Rosaceae family served as a reservoir: for example hawthorns or cotoneasters.

A bit of history

In the early 1970s, the presence of fire blight was observed locally on the western shore of the European continent: Denmark, the Netherlands, Belgium, northern France, etc., probably brought from England by migratory birds.

Specialists predicted that the disease would spread rapidly when the climatic conditions of a year were particularly favorable to it. This was the case ten years later, in 1981, in several pear orchards close to large untended hawthorn hedges. Its extension throughout the country continued in 1982.

A bit of biology

Fire blight infects the aerial part of various plants of the Rosaceae and Malaceae wild, ornamental or fruit families: Crataegus, Pyracantha, Cotoneaster, Stranvaesia, Pyrus, Malus, Cydonia, Sorbus and Eriobotrya. It causes sudden drying out of parts or all of the antlers, which seem to have been burned by a very hot fire.

The bacterium overwinters at the edge of cankers on twigs. In the spring, with the rise in temperature, in humid weather, these cankers emit an exudate: droplets or filaments are disseminated by the wind, rain, animals or personnel circulating in the orchard. The role of foraging bees is controversial. Infections occur via natural orifices: stomata, lenticels, various wounds, and via flowers in the case of pome fruits.

For apple and pear trees, several emission prediction methods have been proposed. They are based on measuring the sum of temperatures above a threshold set at 18.5°C, rainfall and the duration of leaf wetting. In Belgium, as the beginning of bacterial emissions is generally after the end of the main pear flowering period, the risks of floral infections are reduced. But there is still a risk of infection of later secondary blooms. On the other hand, a warming climate could lead to an increase in risks.

Measures to be taken

For our arborists, the psychological impact of fire blight was enormous, since the first measures taken consisted in uprooting or mutilating pear trees which, for the rest, were in perfect condition, after having spent a lot of time throughout the throughout the season to detect the slightest symptoms.

It was necessary to learn to live with fire blight, taking in the orchards as well as in the direct environment different preventive measures not always well accepted or well understood. This is particularly the case with regard to the hawthorn hedges, which were asked to be pruned so that they did not bloom, and not to be uprooted. In addition, the many hawthorns present in nature in certain regions are a permanent reservoir of inoculum. On pear trees, secondary blooms should be removed.

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Another concern came from unmaintained or poorly maintained high-stem pear trees, which were a source of inoculum for a few years, before their complete decline. In ornamental arboriculture, the sale and planting of large-leaved cotoneasters were discontinued.

In chemical control, streptomycin was used which, although very effective, had given rise in the United States to the appearance of resistant strains of bacteria when it was applied frequently (up to 10 treatments per season !). Copper (hydroxide, sulphate, oxychloride, etc.) was well known for its bactericidal effect, and preventive applications during the critical flowering period of pear trees have given satisfactory results. But repeated use of this product can cause a phytotoxic reaction on pear trees, and it can affect the activity of soil fauna. A fungicide, Phosethyl-aluminum, has also been shown to be effective in protecting pear trees during flowering.

Prophylactic measures include disinfecting pruning tools with antiseptics (e.g. Dettol).

Since the 1990s, the use of several antagonistic bacteria, for example Erwinia herbicola and Pseudomonas fluorescens, has been the subject of much research, mainly in the United States, as well as in New Zealand and the United Kingdom.

Sensitivity and resistance to fire blight

The susceptibility of twigs and flowers of current fruit varieties to several strains of fire blight has been the subject of various evaluations. As far as we are concerned, the 'Beurré Alexandre Lucas', 'Beurré Giffard' and 'Beurré Hardy' pear trees, as well as the 'Belle de Boskoop' and 'Golden Delicious' apple trees proved to be the most resistant.

In contrast, the pear trees 'Beurré Durondeau', 'Comtesse de Paris', 'Passe Crassane' and 'Triomphe de Vienne' and the apple trees 'Cox's Orange Pippin', 'Gloster', 'Idared', 'James Grieve', and 'Tydeman's Early' are among the most susceptible. The 'Louis Bonne d'Avranches', 'Légipont', 'Conférence' and 'Doyenné du Comice' pear trees, and the 'Alkmene', 'Elstar', 'Gala', 'Jonagold', 'Melrose' and 'Pinova' apple trees have intermediate strength.

The solution to the problem lies in the creation of resistant varieties of pears and apples. Resistance to fire blight has become an important criterion for selecting new products. At the present stage, the Canadian Research Station at Harrow has released several varieties of resistant autumn pears.

Pseudomonas syringae

The genus Pseudomonas has about sixty species. Pseudomonas syringae is found on a wide variety of host plants, to which correspond different pathovars: pv. syringae on several fruit species, pv. mors-prunorum on cherry trees, pv. persicae on peach trees, pv. actinidiae on kiwis, apricots and sometimes cherry trees. Often associated with it is the species Pseudomonas viridiflava.

In nucleated species, we notice cankers on the branches with emission of gum, then later, after winter, the death of the infected branches; on seeded species, drying of floral bouquets and young twigs, as well as chlorotic spots on the foliage.

The bacterium overwinters in infected twigs, then it is dispersed by rain and wind; it seems favored by wet but cold weather. Infection occurs via natural orifices and wounds: leaf fall in stone species, spring frost in pear trees and kiwis.

The fight consists in protecting the wounds from the risks of infection by copper treatments. They are most numerous after a storm with hail and especially during leaf fall.

Xanthomonas spp.

Several pathovars of Xanthomonas arboricola (= campestris) infect fruit species and cause necrosis of foliage, stems and fruits, or plant wilting following infection of his vascular system.

In hazelnuts, the pv. corylina causes young shoots to dry out in the spring, on young plants or in nurseries. In walnut trees, pv. juglandis produces oily spots on leaves, withering of young shoots, necrotic black spots of bugs and fruit drop. In stone fruits, the pv. pruni causes pitting and leaf drop.

The fight consists of several copper treatments after a long wet period.

Xanthomonas ampelina = Xylophilus ampelinus is the “Disease of Oléron” which infects European vines. It causes elongated brown necrosis on a sector of the vine shoots, and a slow decline. In France, it is a quarantine organism whose declaration to the authorities is mandatory. It seems favored by high soil humidity.

We recommend the disinfection of pruning tools and the treatment of wounds with copper as well as the incineration of pruning wood. After uprooting infected plants, wait several years before replanting.

Other harmful or useful bacteria in arboriculture

Ice-forming bacteria

During late spring frosts, with a negative temperature, the pure water remains supercooled down to -4ºC. Peudomonas syringae present on pear trees acts as crystallization nuclei, with ice formation on the young tissues, then infections and necrosis of the floral bouquets occur via wounds due to frost.

Specific soil fatigue

During the replanting of apple trees after apple trees, the bacterium Pseudomonas fluorescens which has developed on the residues of the roots emits toxic substances into the soil which slow down the growth of new trees. The persistence of the phenomenon is several years.

Bacterial antagonists of bacteria

Ongoing research aims to use bacteria that are antagonistic to fire blight, which would be carried by foraging insects (bees, osmia, bumblebees, etc.) from hives, for example.

The bacterium Bacillus amyloliquefaciens QST 713 is approved to fight against floral infections of Pseudomonas on cherry trees. This bacterium is present in the soil and in flowers.

Bacteria antagonistic to fungi

A strain of Pseudomonas syringae helps prevent fruit rot during storage. Its trade name is “Bio Save 110”.

Entomopathogenic bacteria

The genus Bacillus includes several species used to fight various phytophagous insects: after ingesting foliage carrying Bacillus thuringiensis, the insect dies following the destruction of its digestive system.

Nitrogen-fixing bacteria

Several species of bacteria play an indirect role in the supply of nitrogen to fruit species.

Rhizobium (= Bacillus radicicola) live in symbiosis in nodules that develop on the roots of legumes (= Fabaceae), from the nitrogen supplied to them by these. As these nodules are constantly renewed, those that die release as they decompose nitrogen which returns to the soil: from 100 to 400 kg per ha per year. At the foot of fruit trees, for example, a cover of low-growing white clover can therefore contribute to the nitrogen nutrition of fruit trees.

Azotobacter and Clostridium are bacteria that live freely in the soil; they would also contribute to its nitrogen enrichment.

Bacillus megatherium and B. subtilis fix between 80 and 250 kg of nitrate nitrogen present in the soil during the summer season, per hectare. This prevents this nitrogen from being washed out during the winter. The following spring, when the bacteria die, it will be mineralized and available for crops.

ir. André Sansdrap

Wepion