Phytophthora spp attack the roots and trunks of apple trees and are responsible for crown, collar and root rots of apple trees.
- Phytophthora cactorum is the main species responsible for crown and collar rot.
- Occasionally P. syringae may also cause bark rots but this species is more important as a cause of fruit rot in store.
Crown rot and collar rot are distinct diseases:
- Collar rot is a disease of the scion portion of the tree, affecting bark tissues of the lower trunk at or above the soil line. In the UK collar rot is sporadic and a disease of mature trees
- Crown rot is a disease of the rootstock portion of the tree affecting bark tissues of the root crown region. Crown rot is usually important on young trees in orchard establishment.
- Root rot is a disease of the root system and is usually in association with crown rot but can occur in the absence of crown rot. Root rot is usually important on young trees in orchard establishment.
- P. cactorum and P. syringae cause diseases of a wide range of other economic plants, woody and herbaceous, including strawberry and pear.
- There is evidence for host adapted strains of P. cactorum.
- Most significant is that strawberry crown rot is caused by a specific strain/pathotype.
- P. cactorum is widespread in apple growing regions of the world.
- P. syringae is a significant pathogen of apple only in northwestern Europe.
- P. cactorum and P. syringae occur widely in English apple soils.
Rootstocks vary in their susceptibility to P. cactorum.
- M.9 appears most resistant, but reports suggest regional differences in rootstock susceptibility.
- M.2, M.7, M.26 and MM.111, which are very resistant under UK conditions are susceptible in parts of North America.
- Rootstocks MM.104 and MM.106 and Merton 789 are considered in general to be very susceptible.
- Rootstocks available from abroad also vary in susceptibility to Phytophthora.
- Collar rot is predominantly a disease of Cox and occasionally James Grieve in the UK, but in New Zealand and Europe has been recorded on other varieties including Laxton’s Superb, Ribston Pippin and Ellison’s Orange.
- In the UK collar rot rarely occurs on trees less than 10 years old.
- Cox on M.2, M.9 and M.26 are particularly prone to attack.
Susceptibility of some apple rootstocks to pest and diseases
|Rootstock||Origin||Effect||Phytophthora susceptibility||Fireblight||Woolly aphid||Other|
|M.27||HRI-East Malling||super dwarf||?||?||susceptible||susceptible to winter cold and drought|
|G 65||Cornell, USA||super dwarf||resistant||resistant||?||experimentalsimilar to M.27|
|JM1, 5, 8||Japan||super dwarf||resistant||?||?|
|J-TG-G||Czech Republic||super dwarf|
|M.20||HRI-East Malling||super dwarf||?||?||?|
|P22||Poland||super dwarf||tolerant||?||susceptible||susceptible to drought|
|M9||HRI-East Malling||dwarf||some resistance||susceptible||susceptible||susceptible to winter cold, poor drainage, drought|
|Jork (J)9||Germany||dwarf||resistant||?||?||>hardier than M.9, tolerant soil/environmental stress|
|M8||HRI-East Malling||dwarf||?||?||susceptible||susceptible to poor drainage + drought|
|MAC 9||Michigan, USA||dwarf||>resistance than M9||?||?||susceptible to drought|
|Ottawa 3||Canada||dwarf||resistant||?||susceptible||winter cold tolerant, sensitive to virus|
|P16||Poland||dwarf||resistant||?||susceptible||vigour depends on soil fertility, winter hardy = M.9|
|M.26||HRI-East Malling||semi-dwarf||susceptible||susceptible||susceptible||winter hardy, average drought tolerance|
Susceptibility of some apple rootstocks to pest and diseases – continued
|Rootstock||Origin||Effect||Phytophthora susceptibility||Fireblight||Woolly aphid||Other|
|Bemali||Sweden||semi-dwarf||?||resistant||?||>winter hardy than M.9|
|KSC 28||Canada||semi-dwarf||?||?||?||winter hardy|
|P1||Poland||semi-dwarf||?||?||?||sensitive to virus|
|V7||Canada||semi-dwarf||?||?||?||winter hardy, experimental|
|M.7||HRI-East Malling||semi-vigorous||resistant||resistant||?||susceptible to winter cold, adaptable to soil types|
|MM.106||HRI-East Malling||semi-vigorous||susceptible||susceptible||resistant||average cold tolerance, susceptible to tomato ring spot virus|
|MM.111||HRI-East Malling||semi-vigorous||some resistance||?||resistant||some tolerance of winter cold|
|G30||Geneva-USA||semi-vigorous||resistant||resistant||susceptible||(limited information outside the (USA|
|M.116(AR86-1-25)||HRI-East Malling||semi-vigorous||very resistant||?||very resistant||experimental|
|MM.104||HRI-East Malling||semi-vigorous||susceptible||?||resistant||tolerates dry soils|
|Merton (MI) 793||J.I. UK||vigorous||resistant||?||resistant||tolerant of replant disease|
|Alnarp 2||Sweden||vigorous||?||?||?||winter cold tolerance|
|B54-118||Russia||vigorous||?||resistant||?||winter hardy, drought tolerant|
Usually P. cactorum and occasionally P. syringae are the fungi responsible for crown rot and collar rot in the UK and Europe, but more recently, particularly in other parts of the world, other species of Phytophthora have also been implicated.
- P. cactorum and P. syringae are soil-borne and frequently present in apple orchard soils.
- Primary inoculum may also be introduced into an orchard with infected nursery stock or possibly in contaminated irrigation water.
- Once introduced Phytophthora spp. survives primarily as oospores (resting spores) in plant debris or soil.
- In the absence of the host, oospores may survive in the soil for several years depending on conditions.
- The fungus may also multiply in the soil by colonising fallen apple leaves or fruits on the ground or other plant parts, or even colonising some other plant species, such as certain weed species.
- In wet soil conditions the oospores germinate to produce fruiting bodies (sporangia) which, in turn, release zoospores which move in soil moisture to infect apple bark on roots or scion.
- Disease incidence and development is affected by availability of water above ground and in the soil, soil type, tree age, the height of the graft union, the interaction between scion and rootstock and soil management techniques.
- There is also seasonable variability in the extent to which apple bark tissues are colonised by Phytophthora spp.
- Colonisation by P. cactorum is greatest between the pink bud stage of tree growth and the beginning of extension growth, i.e. when the tree is actively growing in spring/early summer, so wet conditions during this period are conducive to attack by P. cactorum. P. syringae however, is more active in the autumn/winter months when the tree is dormant.
- Infection of apple tissue takes place mainly at the soil surface and tissues at greatest risk of infection are those in contact with the soil or within the range of soil splash.
- Infection of soft-tissue structure such as fine roots can occur directly, but usually wounds are needed for bark infection.
- These may be from frost cracks, cultural operations, or age cracks appearing at the graft union. Infection of root burrs may also lead to crown rot.
- Soil moisture is the most critical factor in crown rot and collar rot and these are therefore more likely to be prevalent on sites with poor drainage.
- Also drainage may be locally poor when trees have been badly planted in post hole bores.
Foliar symptoms are likely to be the first indication of crown or collar rot, but are not diagnostic, merely indicators of root or vascular problems in the tree.
- Affected trees are unthrifty, showing poor extension growth.
- Foliage is often sparse, chlorotic and may show premature colouring in the autumn.
- Fruit is often small and colours prematurely.
- Trees affected by collar rot usually decline progressively over several seasons.
- In comparison, trees with crown rot, particularly in young orchards, usually die quite quickly, especially after a wet autumn or spring.
Collar rot on trunk
Once foliar symptoms have been identified, examination of the scion or rootstock bark may provide more diagnostic opportunity.
- Typically diseased bark is orange – red/brown in colour or dark-brown if the rot is advanced.
- Active collar rot lesions may also be tiger-striped under the bark and may weep on the outer bark surface.
- In extending lesions, the margin between healthy and necrotic areas is indistinct. This is the tissue from which the pathogen is most easily isolated.
- In crown rot the necrosis often extends some way along the main roots and sometimes up to the graft union, but rarely above it unless the scion is a very susceptible variety like Cox.
- In collar rot, the necrosis may extend right up the trunk, into one of the main branches, but does not usually extend below the graft union.
- No fruiting bodies are visible on the lesions. This helps to distinguish it from other wood rotting fungi, but in advanced collar rot, secondary rot fungi may invade the dead tissues.
Other problems that may be confused with crown and collar rot
Crown and root rot are most easily confused with root death/tree death due to waterlogging or wet feet which occurs if roots are waterlogged for significant periods when trees are physiologically active.
- The problem results in similar foliar symptoms.
- If the roots and crown are examined, root death due to waterlogging usually shows as dark brown/blackened roots rather than the distinct orange red/brown colour of Phytophthora.
- In addition, there is a sour smell rather than the alcoholic smell associated with Phytophthora root rots.
- Waterlogging may also result in bark necrosis but this is probably caused by diffusion of toxins brought up in the water conducting tissues, rather than by fungal activity.
- Such bark necrosis is also associated with deep staining in the central woody part of the trunk which may extend for some way up the tree.
- This symptom is not typical of Phytophthora diseases.
Collar rot, particularly when lesions are well advanced, may be confused with other wood rotting fungi, such as some of the bracket fungi.
- However, examination of the tissue under the bark, especially the wet orange/red-brown rot, usually distinguishes collar rot.
- Check young orchards on susceptible rootstocks in spring and late summer/autumn looking for trees that fail to leaf out or that do develop autumn colours earlier than normal.
- Examine the rootstock below ground of such trees for signs of bark rot.
- Phytophthora diseases are rather unpredictable, being dependent on wet soil conditions.
- However, mature Cox orchards are most susceptible to collar rot and trees can be saved if the initial symptoms are spotted early on.
Check orchards at risk in late June and look especially at the graft union for damp patches.
- Look for pale yellowing leaves on the whole tree or on a single scaffold branch.
- Once a diseased tree has been identified it can be spot treated.
- Do not plant trees on susceptible rootstocks on wet sites. Ensure that drainage is adequate prior to planting. Selecting the right site initially is important.
- Trees are most at risk from crown rot when they are young and are being lifted from the nursery or planted out as this is when damage can occur.
- Trees should not be lifted or planted in wet conditions as rainfall increases the risk of infection.
- Plant trees carefully. Avoid creating locally poor drainage around newly planted trees.
- Planting high grafted trees (minimum 40-70 cm above soil level), or trees with resistant interstocks, will ensure that susceptible scions are less prone to infection by soil splash.
- In mature orchards, keep tree trunks clean of weeds. Check regularly that rabbit guards are not too tight. Keep soil away from the graft union and avoid mechanical damage to the trunk.
- Remove fallen fruit from the base of the trees and into the alleyway where they can be pulverised. Fallen fruit provide a substrate for regeneration of P. cactorum.
- In addition, encourage rapid leaf decomposition in the autumn by use of post-harvest urea sprays and regular mowing to macerate leaves.
- Fallen leaves are the main substrate for P. syringae multiplication.
- Some research into biological methods of control of crown and collar rot has been carried out, but there are no commercial methods available at present.
For crown rot, most damage is already done by the time foliar symptoms are evident. Therefore, corrective treatments are not usually worthwhile.
- For crown rot, trees showing foliar symptoms are usually too badly damaged to save. These should be grubbed and burnt.
- The replanted tree and the trees in the rest of the orchard can be treated with a copper product (check status of current authorisation) or the chemical fungicide Fubol Gold WG (mancozeb + metalaxyl).
- Fubol Gold should be applied post harvest but before green cluster stage.
- Where collar rot lesions are not too advanced, control can be achieved by cutting out all bark in the affected area, including at least 5 cm of healthy tissue to expose the sapwood.
- The cut area must also extend through the graft union.
- Alternatively, to save labour, a groove is cut down to the sapwood in the healthy tissue around the lesion, so it surrounds the lesion and extends through the graft union.
- Physical acting products such as BlocCade when applied to pruning wounds or cut surfaces can help to protect against further infection by fungi such as Phytophthoras. To be effective, the wounds must be treated immediately after pruning.
- Applying fungicides as a trunk spray or to the orchard floor is ineffective.
Harris, D C (1991). The Phytophthora diseases of apple. Journal of Horticultural Science, 66, 513-54