Phytophthora (phyto= plant; phthora= destroyer) species are serious plant pathogens in natural and man-made environments worldwide, the most infamous species being Phytophthora infestans which causes potato rot and was responsible for the famine and subsequent mass emigration of the Irish to America. Another example of a devastating species is P. cinnamomi, which causes plant diseases on all continents (except Antarctica) and is listed in Australia as a 'key threatening process' under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).
Often incorrectly called 'fungus' due to the very similar appearance (an example of convergent evolution), they are actually more closely related to brown algae and diatoms. The most appropriate common names are 'water mould', 'root rot' or 'dieback'.
There are two main classes, airborne foliar Phytophthora species (e.g. P. ramorum causing significant damage in North America and Europe; currently still absent in Australia) and species that are soilborne, which attack roots, trunks or branches near ground level. Consequently, infected plants developing root rot or being girdled around the trunk will have problems of water and nutrient uptake.
The most common species found in Western Australia are Phytophthora multivora, P. arenaria, P. cinnamomi, and P. spec. ohioensis. This is surprising as it was previously thought that P. cinnamomi would be the most widely distributed species, but this seems not to be the case for the suburbs of Perth.
A survey by Davison et al. (2006) revealed that Phytophthora species enter Western Australia via nursery stock from interstate, but little is known regarding survival outside the nursery setting. Of particular interest is P. palmivora, a tropical species often causing problems in cacao, coffee, papaya and other tropical crops. This species was detected in the above survey and was for the first time recovered in Perth in a planted situation (Barber et al. 2013).
It is important to mention that other diseases can show very similar plant symptoms, such as several species of the Seiridium fungus which cause cypress canker. The only certainty of positive identification comes from a diagnostic test. To detect Phytophthora from samples, it is crucial to collect roots, soil and bark at the right stage (early onset of symptoms) and in the right season (warm and moist conditions in spring, autumn and winter).
Despite the diagnostic necessity for confirmation, below are typical Phytophthora symptoms which are all related to the consequences of root/stem damage and therefore disturbed water and nutrient uptake. Depending on plant species and conditions, not every symptom will develop.
Crown symptoms: Reduced foliage and dieback of branches give the appearance of a sparse canopy.
Leaf symptoms: Yellowing or other discolouration, drying of leaves (or becoming mushy for succulent leaves), droopy leaves and leaves of much smaller size.
Stem symptoms: Bleeding as a wound response and/or brown discolouration (=lesions) of cambium underneath the bark (cells being killed by Phytophthora). There is a clear border between tissue not yet invaded (green plant tissue) and infected areas. Lesions can be water soaked.
Root symptoms: Overall reduction of the root system (especially fine feeder roots), brown discolouration, soggy roots.
Fruit symptoms: Fruits of much smaller size than normal, shrivelling of fruits. If fruits hang low to the ground and are subject to (contaminated) water splash, they can be infected and develop discolouration and rot.
Abiotic symptoms: Due to the inability to take up water, soil around large infected trees is wetter than the soil around healthy trees.
Phytophthora species are characterised by a very fast asexual lifecycle. It produces an abundance of swimming zoospores which are released from spore sacs called sporangia. Once these adhere to plants they germinate and produce hyphae which grow through the plant. Rapidly or eventually, nutrients in invaded plant cells are extracted and the cells die. Throughout the process more spore sacs with zoospores are formed and released, ready to actively swim to new roots. Once environmental conditions become unsuitable, most Phytophthora species are also capable of producing long lasting (probably 3 to 5 years) survival structures such as chlamydospores and oospores.
The spread of the disease can be several metres per year by actively swimming zoospores and root-to-root contact in well-drained areas, but is drastically increased in waterlogged locations, on slopes by water run-off or by dispersal of contaminated soil or plant material from one location to another.
There are four basic management principles.
1) Prevention – Protect Phytophthora-free areas from introduction of the pathogen.
Of course, the best case scenario is absence of Phytophthora and to prevent introduction into the garden. Several cautionary principles will help to reduce the risk of introduction:
a) Sourcing of plants: Choose nurseries committed to a good hygiene, which includes
- use of above ground benches with metal mesh or pots sitting on blue metal (This ensures that pots are free-draining and water splash does not reach other pots, so no zoospores can contaminate pots should there be a source contamination).
- use of Phytophthora-free irrigation water and clean potting mix.
Nurseries and Garden Centres can be part of the voluntary Nursery Industry Accreditation Scheme Australia (NIASA). Accreditation is given and maintained through external auditors verifying hygiene standards are met, which consequently guarantees that purchased plants are free from Phytophthora. Look for the NIASA logo or ask nurseries whether they are NIASA accredited.
b) Sourcing of potting mix and mulch
When you are buying potting mix, look on the package for the symbol with the ticks and the words “Australian Standard” or even “Australian Premium”. This ensures that the company gets independently audited on a regular basis and meets the requirements of a good quality potting mix in several aspects which also includes freedom from Phytophthora. The same applies to packaged mulch.
For bulk quantities the appropriate storage becomes another important point. Stockpiles on bare ground near contaminated bushland can potentially harbour the pathogen. In regard to mulch, many councils and tree loppers are "dieback aware" and take appropriate precautions to avoid contamination.
c) Avoid unnecessary traffic in your garden
Especially when driving on unsealed roads during wet weather, mud sticks to vehicles and can be a source of contaminating your property with Phytophthora. You will greatly reduce the risk by washing down your vehicle before entering, or if you park where there is no water run-off into the garden. Source of contamination can be also dirty tools or boots, so sterilising in a bleach solution or a water bath with a special disinfectant from a horticultural supplier is recommended.
2) Improve environment to reduce impact
In the unfortunate event Phytophthora is present in the garden, it is possible to at least reduce its impact. Even for fruit trees very vulnerable to the disease (eg. avocado), trees can be kept alive and productive if the environment is improved.
For Phytophthora and plant diseases in general, a major factor influencing disease severity is the environment. Simply, the better the environment for the plant, the lower the devastating effect of the pathogen. In gardens, we can manipulate some of these properties to create an environment favourable to plant health, and also make it less favourable for the pathogen. Some of these effective measures which help to manage the disease are:
Some soils are already suppressive to Phytophthora, partly due to a naturally occurring microbial community antagonistic to the pathogen. Unfortunately, this is in general not the case for the relatively poor soils in the South-West of Western Australia.
3) Choice of plants
Plant response to Phytophthora can be categorised as: resistant, tolerant or susceptible. Many herbs and some fruit trees are resistant or at least tolerant to the disease, whilst other plants are easily killed. Just by selecting fruit trees less vulnerable to dieback, you are able to manage Phytophthora in your garden. Examples of fruit trees able to withstand Phytophthora are mango and mulberry, whilst avocado and papaya are examples of very susceptible trees. However, intensive breeding for major fruit crops has resulted in Phytophthora-tolerant rootstocks for susceptible plants such as avocado, apple, pear, citrus and others.
Plant species ideally suited to our climate and soil will grow more vigorous and healthy than plant species which are marginal and less suited. Accordingly, there is less disease pressure in fruit trees which are good matches with our Mediterranean environment in South-West Western Australia such as citrus, figs, mulberries, olives, etc.
Whilst there is sufficient knowledge of the susceptibility of fruit trees and Phytophthora cinnamomi and the more common Phytophthora species, very little is known about others such as P. multivora, P. arenaria and their effects.
4) Eradication of Phytophthora
Due to the survival structures of the pathogen, eradication is very difficult to achieve. Fumigation of infested areas is not possible for the home garden. The option of "starving the pathogen to death" by keeping an infested area completely vegetation-free for a minimum of 4 years is not practicable. It has been shown for Phytophthora cinnamomi in avocado orchards that soil solarisation will effectively reduce the pathogen population and therefore extend the life of the trees in the orchard.
Barber et al. (2013) A diverse range of Phytophthora species are associated with dying urban trees. Urban Forestry & Urban Greening 12, pp. 569-575.
Burgess et al. (2013) Next Generation Sequencing reveals unexplored Phytophthora diversity in Australian soils, Centre for Phytophthora Science and Management, Murdoch University.
Davison et al. (2006) Pathogens associated with nursery plants imported into Western Australia. Australasian Plant Pathology 35, pp. 473-475