Bacterial pathogens cause financial losses to fruit growers worldwide —South Africa is no exception. By Anna Mouton.
The genera Pseudomonas and Xanthomonas contain disease-causing species that attack many different plants. But whereas all Xanthomonas species are associated with plants, Pseudomonas species have more diverse lifestyles.
Pseudomonas bacteria responsible for blossom blast, cankers, and gummosis in stone fruit fall in the Pseudomonas syringae complex. Calling it a complex is apt — scientists are still trying to figure out the complicated relationships between its different species and variants and their plant victims.
Cankers and die-back of stone-fruit trees were common in the Western Cape as early as the 1920s. Pseudomonas syringae was identified as the cause of die-back in apricots in 1960. Subsequently, Pseudomonas species were associated with stone-fruit cankers, apple blister bark, pear blossom blast, and cherry leaf spot.
Recent Hortgro-funded surveys concluded that Pseudomonas syringae pv. syringae and Pseudomonas viridiflava are responsible for bacterial cankers in Western Cape plum orchards. To keep it simple, this article will refer to Pseudomonas syringae.
Like Pseudomonas syringae, Xanthomonas arboricola is a species complex that includes important pathogens of stone fruit — Xanthomonas arboricola pv. pruni — and nuts — Xanthomonas arboricola pv. corylina and pv. juglandis. This article will refer to the entire complex as Xanthomonas arboricola.
Black spot disease caused by Xanthomonas arboricola was first observed on Western Cape stone fruit in 1956. Although the disease mainly affects fruit and leaves, stem cankers and die-back sometimes occur.
Stone-fruit pathogenic Pseudomonas and Xanthomonas species plague all major growing regions worldwide. Their economic impact results from reduced fruit yields and quality, and tree deaths.
Pseudomonas can kill 10%–75% of young trees. Destruction of buds and flowers can reduce yields by 10%–20% — losses of up to 80% have been reported. Older orchards severely affected by cankers may be unprofitable, leading to their removal.
Canadian researchers identified Pseudomonas syringae as the cause of die-back in pome-fruit rootstocks. In some cases, 60% of M.9 rootstocks were lost. Die-back of Old Home pear rootstocks occurred in one nursery. In South Africa, M.793 rootstocks are known to have suffered blister bark due to Pseudomonas syringae infections.
A 1989 publication estimated that the annual damage to South African stone-fruit crops from Pseudomonas species exceeds USD 10 million. Research from 2010 stated that Xanthomonas arboricola epidemics in Italy could lead to annual losses above EUR 10 000 per hectare in modern commercial plum orchards. Australian stone-fruit losses are claimed to reach AUD 3.1 million during outbreak years.
Although Xanthomonas arboricola occurs in the European Union, it is on their list of quarantine pathogens as part of a strategy to minimise its spread.
How do I recognise these diseases?
Pseudomonas syringae cankers in stone-fruit trees usually develop at graft unions, pruning wounds, and the base of spurs. Externally, cankers appear as slightly sunken and darker brown areas of bark. Internally, tissues are discoloured orange to brown. Most cankers exude gum, especially in spring.
Cankers can girdle entire limbs, killing the section above the canker.
Pseudomonas syringae also causes dormant bud blast destroying large numbers of flower and leaf buds, and resulting in poor bloom, especially in pears, apricots, and cherries.
Leaf infections start as small water-soaked spots that turn brown and dry before falling out to create holes. Dark surface spots with underlying gummy or spongy tissue characterise fruit inf.
In bacterial blister bark of apples, affected areas are raised, not sunken, and fruit spurs are often killed. Pears can develop blossom blast in cold, wet weather.
Xanthomonas arboricola bacterial spot also starts as water-soaked leaf spots that may fall out, lending leaves a shot-hole appearance. Early leaf drop can occur. Fruit develops dark spots, as described for Pseudomonas syringae, and may exude gum.
Sometimes Xanthomonas bacteria invade stems, form cankers, and cause die-back.
The take-home is that Pseudomonas and Xanthomonas infections can look the same. Many other microbes can also cause seemingly identical cankers and leaf spots. Suspected cases should therefore be confirmed by laboratory testing offered by the Plant Disease Clinic at the Department of Plant Pathology of Stellenbosch University.
How do these bacteria spread?
Pseudomonas syringae and Xanthomonas arboricola overwinter in infected tissues and dormant buds, enabling them to rapidly colonise new leaves in spring. The bacteria initially grow on the leaf surfaces without causing disease. Cool, rainy weather, high humidity, and wind promote their multiplication and dispersal.
Spread occurs through anything that carries the bacteria, for example, splashing water, windblown rain, pollinating insects, and pruning tools. Pseudomonas syringae is also seed-borne and found on herbaceous weeds, including perennial pigweed, common sow thistle, and yellow oxalis — surings — in Western Cape stone-fruit orchards.
Pseudomonas syringae and Xanthomonas arboricola invade plant tissues through openings such as stomata, leaf scars, and wounds. Pseudomonas syringae even takes advantage of holes from insects such as leaf miners. Overhydration of tissues by heavy rains facilitates infections.
During the growing season, bacteria continue to multiply and spread when environmental conditions are favourable. Hot, dry weather tends to suppress their numbers.
Cankers usually start when bacteria enter through wounds or the bases of infected buds or spurs in autumn or early winter. The bacteria invade the vascular tissues and break down host cells. Canker development slows when trees come into active growth in spring, as trees form callus to wall off the infected areas. However, cankers can flare up again during tree dormancy.
Inactive infections, such as non-visible cankers in nursery trees, introduce Pseudomonas syringae and Xanthomonas arboricola to new orchards.
Control of Pseudomonas and Xanthomonas
Stone-fruit cultivars differ in their sensitivity to bacterial infections. In general, plums bred by the Agricultural Research Council and managed by Culdevco are better adapted to local conditions, and therefore healthier and less disease prone. Some local cultivars nonetheless have poor disease tolerance, and no cultivars are completely resistant.
Certain stone-fruit rootstocks are associated with greater bacterial disease susceptibility — consult the detailed tables in the September 2019 issue of Fresh Quarterly for more information.
Suboptimal conditions such as drought and waterlogging predispose trees to disease, especially when coupled with nematode and fungal infections. Apparently healthy but infected nursery trees readily develop disease following transplant stress.
Avoiding large pruning cuts during wet weather and using registered wound sealants provide some protection in orchards. Growers should take care not to spread infections when pruning to remove cankers. Do not use contaminated tools on healthy trees and destroy infected plant material.
Registered products for controlling Pseudomonas and Xanthomonas in stone fruit are available in South Africa. Most are copper-based compounds. Spraying aims to kill bacteria on the plant surface before they invade tissues. Be aware that incorrect use of copper compounds can lead to the development of resistant bacteria.
Chemical treatments will not cure trees that have active cankers. As for many fruit-tree diseases, starting with uninfected plant material is the best control strategy.
Image: Gummosis caused by Pseudomonas syringae in a cherry tree.
Supplied by Yolanda Petersen | Agricultural Research Council.