Phytophthora diseases of cutflowers

Page last updated: Tuesday, 9 December 2014 - 12:26pm

Please note: This content may be out of date and is currently under review.

Disease management

It is extremely difficult to eradicate Phytophthora from an infested site, especially when perennial crops are involved. If it is known the disease is limited to a particular area, or if the crop is being grown in soil-less media in bags or containers, then it may be possible to remove the soil or media.

In open ground, try to remove soil to as great a depth as possible. While the spores may be predominantly in the root zone, they may have been carried to a metre or more deep in soil water. The more soil that can be removed, the better.

Soil or media may be treated to reduce the pathogen. The difficulty is achieving treatment at depth. Most soil sterilants only penetrate to about 30cm deep at best.

Steam is a chemical-free option but treatment in the field relies on having the correct equipment, access to power and a suitable water supply. The use of steam permits pasteurisation which uses a slightly lower temperature and so allows a range of “friendly” microflora to remain.

The presence of beneficial microflora usually helps to limit the growth of pathogens if and when they are reintroduced, as opposed to chemical fumigation which eliminates both harmful and beneficial organisms.

Chemical sterilisation also uses highly toxic chemicals and specific permits may be required for use. Methyl bromide used to be the fumigant of choice but due to its deleterious effect on the ozone layer is now banned except in well defined circumstances (such as for quarantine use).

Other alternatives are metham sodium, chloropicrin, Basamid® and Telone®C35 (where nematodes are also an issue). With continual use over time, control problems may develop.

Rootstocks

Resistant rootstocks, if available, allow growing of susceptible species on an infected site, or reduce the risk of losses. Some native plant species can be grafted onto stocks of the same or a related species known to be highly tolerant of Phytophthora

Some selections of waxflower have a high degree of resistance to Phytophthora and can be used as rootstocks. This option may not be available for all plants.

Growing method

Another alternative, suitable for many exotic cutflowers, is to grow susceptible species in soil-less media or hydroponics. Plants in bags or containers, if infected, are easily isolated from the rest of the crop and can be disposed of and burnt.

Hydroponics does not necessarily eliminate the risk of Phytophthora. The disease may enter on infected plant material, blow in with contaminated dust, or spores. Depending on the method of infection and how the system is set up, infection may spread rapidly throughout, or may be confined to one section.

Biofumigation

Biofumigation can be used as part of a disease management package. It cannot be relied on to eliminate the pathogen.

Brassica species have a suppressive effect on some soil pathogens and pests. Mustard (Brassica juncea) shoot material totally suppresses the growth of P. cinnamomi isolates under laboratory conditions. However, efficacy in the laboratory does not necessarily translate to the field. In addition, while sporangia production is suppressed, there is no effect on chlamydospore production.

Other variables may include time of year, time of incorporation, incorporation method and crop agronomy. The optimum time to incorporate brassica plant material is just after flowering when it can be more effective due to the levels of glucosinolates in the plant.

Other work has evaluated Acacia, Kennedia and other native legumes. Acacia pulchella has been shown in field and glasshouse trials to protect B. grandis from Phytophthora infection. It suppresses the growth of the fungus in the soil. Other Acacia species are less effective or ineffective.

Research has demonstrated that exudates from the roots of Acacia pulchella are associated with high levels of lysed (broken or burst) Phytophthora chlamydospores.

Soil solarisation

Soil solarisation is a relatively cheap method that can be effective. It involves using clear plastic to tarp cultivated, moist soil for four to six weeks in summer. The soil temperature rises to above 60ºC and kills a range of pathogens (including Phytophthora), but leaving most beneficial microflora intact in a similar manner to pasteurisation with steam. 

Limitations include achieving sufficient heat at depth and the logistics of applying plastic to large areas. The method can also be applied to potting mix.

Composts and mulches

Composts and mulches, especially marri, karri and other hardwood bark, are highly suppressive of Phytophthora after composting. On a broadacre scale, composted mulches can be used for weed suppression and may assist with soil moisture retention during summer.

Limitations include the logistics of transporting, spreading and incorporating large quantities of such material, and cost. The materials may need to be reapplied for continuing benefit. Growers also need to be sure they do not import disease in contaminated material. The use of composts may be incompatible with some plant species, e.g. banksia.  This control method may be more suitable for exotic cutflower crops grown in soil-less media containing a high proportion of composted bark.

Other biological control

Various microbial preparations have been tried, however results are variable. Richoderma does have a suppressive effect on mycelium but has also been shown to stimulate the production of oospores. As with all microbial preparations, ensuring high viability of microbes in packaged form is critical.

Water management

Good soil water management is especially important to prevent Phytophthora infection. Both waterlogging and drought will stress plants, making them more susceptible to disease. Often good irrigation practice can eliminate most of this. 

Scheduling with instruments such as tensiometers is critical. Root infection only occurs when soil has been saturated for a period of time, which varies with soil type. Root infection has also been recorded at depths greater than 2m. Where soil has a compacted layer, ripping may be useful. Mounding beds can also help in some situations.

Other factors

Phytophthora infection may frequently be present, but if the rate of root replacement keeps up with the rate of root death, then the disease may not be noticed. Salinity can exacerbate the disease. For example, in chrysanthemum, the rate of P. cryptogea infection increased from 20% to 80% when plants were stressed.

Wounding or other tissue damage such as from frost or nematode infection also aids infection. In one study, the rate of infection in lucerne tripled when root-knot nematode was present.

Excessive nitrogen can make plants more susceptible. Manipulation of pH and calcium levels can have a positive effect in some cases.

Contact information

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080