News Plant Pest(s): Pseudomonas syringae pv. syringae in pear and stone fruit
What is Pseudomonas syringae (Ps) and where does this bacterium occur?
Pseudomonas syringae, or Ps for short, is a complex of bacteria consisting of more than 60 variants, each of which can cause disease in specific plant species. Those variants are called pathovars. They are found in all climate zones and on all continents, but are most at home in temperate climates with wet and freshwater sources such as in Belgium. We find Ps bacteria on many plant species, including several agricultural and horticultural crops. Moreover, they are also sometimes found in rivers, lakes, clouds, rain and snow, making them closely associated with the Earth's natural water cycle.
In this edition of the Plant Pest(s) , we highlight the most cunning pathovar of the Ps complex: Pseudomonas syringae pv. syringae (aka Pss). Pss was first isolated in the Netherlands from lilacs in 1902, but damage by Pss has since been established in fruit trees, vegetables, ornamental plants, cereals and grasses. Especially in fruit orchards, Pss has been a recurrent problem in recent years, with blossom death in pear trees and bacterial canker in stone fruit as typical disease symptoms. The pathogen can be easily isolated from diseased plant tissue in the lab. On culture medium, bacterial colonies initially appear unremarkable (grayish-white color), but under UV light they usually fluoresce blue (Figure 1).
Pss infection process and symptoms
When leaves fall in autumn, leaf scars provide an ideal entry point for Pss. Through these scars, bacteria migrate to dormant buds, where they overwinter. Pss can also survive the winter in affected woody tissue, better known as cankers. In spring, the bacteria multiply and spread through rain and wind to budding leaves and blossoms. Here the bacteria survive epiphytically, waiting for the right conditions to invade and sicken the plant. Prolonged periods of cool and humid weather especially promote disease progression. In addition, injuries from hail or frost make trees especially susceptible to infection by the pathogen.
The mid-April 2024 night frost combined with the cold and damp spring weather contributed to severe Pss infections in pear orchards.
The first symptoms usually manifest as the death of leaf and flower buds and the wilting of blossoms and young shoots. In summer, spots may appear on the leaves and fruits. In pear, these leaf spots are brown-black (Figure 2), while in stone fruit they are initially watery and later turn reddish-brown, sometimes with a yellow edge (Figure 3). Fruit damage is most pronounced in pear (brown-black spots). Fruits with symptoms often drop off during the June molt. If they do persist, they show typical curvature or coarsening of the fruit skin, resulting in loss of quality. In stone fruit (not pear), infection can also progress into the woody tissue of the tree, leading to canker formation and the death of entire branches. Cankers can be recognized by black discolored spots on the bark, sometimes combined with a sticky, colored discharge (gum formation). An orange discoloration is often visible under the bark.
Pss symptoms in pear are sometimes confused with those of fire blight, caused by Erwinia amylovora. An important difference is that Erwinia infections occur mainly at higher temperatures. As a result, symptoms are usually observed later in the season than those of Pss.
Did you know that Pss has superpowers?
Special membrane proteins of the bacteria can promote the formation of ice crystals at temperatures above the natural freezing point of pure water. This is the so-called “ice nucleation” or IN-activity of Pss. Thanks to this property, the bacteria can influence the formation of precipitation droplets and thus regulate its own deposition from the atmosphere. Clever and convenient right?
So can humans exploit this IN-activity to control the weather or make snow, for example? The answer is “YES! Among other things, additives based on the bacterial IN protein are being added to artificial snow to use snow cannons to keep ski slopes covered at higher temperatures.
Unfortunately, Pss also uses its IN forces to damage plants. The bacteria can freeze plant tissue at temperatures just below freezing. Most woody plants can tolerate freezing temperatures down to -5 °C without much problem, but with Pss, they can suffer frost damage much faster. This allows the bacterial cells to penetrate more easily to gain access to nutrients. Another strategy used by Pss to colonize plant tissue is the production of coronatin, a toxin that regulates the opening of stomata. Stomata are natural openings on the surface of plants that serve for water and gas exchange with the air. Normally, these stomata close spontaneously when a pathogen tries to invade. Coronatin goes against this defense mechanism and causes the stomata to reopen to allow Pss free access.
What can you do to prevent Pss infestation?
Fighting Pss is unfortunately not possible. In Europe, the use of antibiotics as plant protection products is not allowed. So the only good strategy is to prevent infections or minimize the risk of infection. How? First of all, it is important to buy healthy propagating material (free of bacteria). In addition, irrigation on above-ground plant parts should preferably be avoided as much as possible, as well as inflicting unnecessary injuries that are the main entry points for the bacteria. Since infections occur mainly under wet conditions, it is also advisable to prune only during dry periods.
When the weather is (near) freezing, it is very important to protect fruit trees and fruit blossoms from frost with heat or other methods. For example, short cut grass under the trees can already cause the soil to give off its heat better at night, so the ambient temperature around the trees will be slightly higher. In addition, good mowing management and weed control is important because grasses and weeds are often a source of Pseudomonas. Other potential sources of infection, such as infected branches, should also be consistently removed from the orchard.
Finally, products can be used to inhibit the growth of Pss. There are some copper-based agents recognized that are applied at high-risk times for infections by Pss, such as in spring and at leaf fall. There are also alternative products based on bio-control organisms (e.g. Aureobasidium pullulans, Bacillus subtilis, B. amyloliquefaciens).
How is ILVO helping with Pss?
ILVO is involved in several aspects of Pss research. We are partner in the VLAIO project 'Ctrl-Pseudo' in which the population dynamics of Pss in Flemish pear and cherry orchards are monitored. Also within Ctrl-Pseudo, conditions that promote infections are being mapped, and reliable greenhouse infection tests are being developed to evaluate new control methods against Pseudomonas.
Furthermore, a doctoral project is investigating a bio-control strategy against bacterial cancer in cherry based on bacteriophages. These are viruses that can destroy highly targeted harmful bacteria.
Finally, ILVO enables the export of pears by performing the mandatory analyses of bacteria (and insects). Belgium is among the top pear producers in the EU, with the flagship pear being the Conference pear. Before 2014, 60% of production was exported to Russia. However, on Aug. 7, 2014, Russia implemented a boycott on European food and alternative markets had to be sought. New markets were tapped in China, Brazil and Vietnam, among others, but these countries impose specific requirements about the health of the pears. For example, for export to Vietnam the absence of Pss must be demonstrated. For this purpose, we analyze the crown on the bottom of the pear (these are the dried flower parts) for the presence or absence of the bacterium (Figure 4).
