Phytophthora

The name Phytophthora literally means plant destroyer — fitting for a genus that abounds in disease-causing species. By Anna Mouton.

Phytophthora infestans thrust the genus into notoriety by launching the Irish Potato Famine of 1845–1849, estimated to have killed 1 million people. Phytophthora infestans was first described in 1975, and the genus Phytophthora has since expanded to approximately 170 species — more come to light yearly.

Most Phytophthora species are pathogens — disease-causing agents — of plants but some attack invertebrates and fish.

The economic impact of Phytophthora is staggering. Phytophthora infestans alone is reported to cause annual global losses in excess of USD 6.7 billion. Phytophthora sojae is responsible for annual global soya-production losses of USD 1–2 billion. And Phytophthora cactorum costs Canadian fruit growers about CAD 2 million per year.

Phytophthora cactorum was first identified in South Africa in 1973 when it was discovered causing die-back of mature apple trees. Phytophthora cactorum crown and root rots had previously been described in apple trees from North America all the way to New Zealand.

Apple crown and root rots in South Africa remain associated with Phytophthora cactorum — several other species of Phytophthora affect apples but have not been found here. Other species of Phytophthora also plague forest and fruit trees worldwide.

A 2009 Hortgro-funded project led by Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University concluded that Phytophthora cactorum is also one of the soil organisms involved in apple replant disease. This finding aligns with international research.

How do I recognise this disease?

Phytophthora attacks the lower parts of the tree leading to death and decay of fine roots — called root rot — or rootstock damage just below the soil surface — called crown rot. Collar rot occurs when Phytophthora girdles the scion. Trees may suffer one or more of these simultaneously.

Infected roots may be dark brown or orange and visibly rotted. Trunk infections manifest as brown or orange discoloration of the ordinarily green cambium layer below the bark in apples and brown or cream mottling in stone fruit. Collar rot may form a sunken discoloured area resembling a canker low down on the scion.

Stone fruit with Phytophthora root and trunk rot may exude gum from the bark at the foot of the tree.

All the above forms of rot impact the uptake and transport of nutrients and water. The degree of damage will determine the signs shown by the tree. Leaves of infected trees may discolour earlier in autumn, while extensive root death and advanced trunk girdling are associated with yellowing or wilting leaves and cessation of shoot growth. Severe damage will bring on shoot and branch die-back, and yield and fruit-size reductions.

Root rots can smoulder for years while affected trees gradually decline. Signs include poor growth and early leaf fall. Crown rots often develop faster and can sometimes kill trees within months.

Young apple trees are especially sensitive to crown and root rots because their root systems and crown areas are smaller.

The signs of Phytophthora rots look similar to those of many other diseases. Suspected cases should therefore be confirmed by laboratory testing, which the Plant Disease Clinic offers at the Department of Plant Pathology of Stellenbosch University.

How does Phytophthora spread?

Phytophthora species are oomycetes — organisms known as water moulds that superficially resemble but are unrelated to fungi. Oomycetes are more closely allied with algae such as kelp than with fungi. They tend to be mistaken for fungi because both form threadlike growth and infective spores. However, their biological differences matter when it comes to chemical and other control measures.

Root-rot Phytophthora species can survive for years as resting spores in the soil, even without plant material. Swimming spores — called zoospores — are released under wet conditions. They follow chemical or electrical signals from potential host plants, making their way along films of soil water to initiate new infections.

Infected nursery trees and contaminated soil and water can bring Phytophthora into orchards. Several studies have reported that Phytophthora species — including Phytophthora cactorum — occur in rivers and other open surface irrigation water.

In general, Phytophthora organisms prefer loam or clay over sandy soils and are more abundant in soils with low organic-matter levels. Mediterranean climates seem to promote severe Phytophthora root and crown rots due to relatively warm soils that seldom become excessively wet. Although Phytophthora species require soil moisture, they also need oxygen, so numbers drop in saturated soils.

Control of Phytophthora

Preventing the introduction of Phytophthora is by far the best control strategy — establish orchards with healthy nursery trees and avoid transferring contaminated soil between infected and uninfected orchards.

Contaminated irrigation water is a potential source of infection, although a lesser risk than infected plant material or soil. Removing Phytophthora from irrigation water is not cost-effective for orchards or open-field nurseries. Water used to irrigate containerised trees should be monitored and disinfected to prevent Phytophthora introduction.

Phytophthora crown and root rots are nearly synonymous with wet soils. Good drainage is essential, and all soils benefit from ridging. Correct irrigation scheduling will minimise wet periods during which zoospores can move through the soil and locate new hosts.

A Canadian study compared microjet and drip irrigation in the tree row to full-surface sprinkler irrigation in a one-year-old apple orchard on sandy loam soil. Drip-irrigated trees were least affected by Phytophthora cactorum crown and root rot.

High soil organic-matter levels are associated with fewer Phytophthora infections. This is no doubt partly because organic matter builds better soil structure with less compaction and improved drainage. There is also evidence that a robust soil microbial population can repulse pathogenic Phytophthora — another reason good soil health makes good sense.

Mulch and compost applications are one component of an integrated management strategy against soilborne Phytophthora diseases. Another approach is calcium supplied as gypsum or calcium-nitrate soil applications.

Australian avocado growers regularly irrigate with 0.5 tonnes per hectare of micro-gypsum, provided soil potassium and magnesium levels are sufficient. Calcium is mildly toxic to Phytophthora and improves soil aeration and drainage in the long term.

There are currently no chemicals registered for the control of Phytophthora crown and root rots in pome-fruit orchards in South Africa, but a mefenoxam soil application is registered for Phytophthora crown and root rots in stone fruit.

Although phosphonates are used in other tree crops and apple trees in other countries, they are not registered for Phytophthora control in apples in South Africa.

Bonus: Apple rootstock research

Trials assessing the Phytophthora resistance of apple rootstocks have yielded inconsistent results. The traditional Merton and Malling-Merton rootstocks vary in susceptibility: M.25, M.26 and MM.106 are the most sensitive, M.9 is considered moderately tolerant, and M.7 varies from moderately to highly susceptible in different regions.

Phytophthora tolerance is one of the selection factors for the second-generation Cornell-Geneva rootstocks, but no rigorous studies compared their tolerance to that of the Merton and Malling-Merton rootstocks.

North American trials found G.11, G.41, G.65, G.202, CG.3041, and CG.5935 to be tolerant against Phytophthora diseases. However, a recent study in Korea reported that G.202 and G.214 were as susceptible as M.26 and more susceptible than M.9 to crown canker caused by Phytophthora cactorum.

To further complicate matters, a 10-year trial of mature apple trees found that the scion cultivar affected rootstock susceptibility. The susceptibility of apple rootstocks may also vary according to the Phytophthora species causing the disease.

Hortgro is currently funding research comparing the Phytophthora tolerance of eight Geneva rootstocks to four widely planted rootstocks. Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University leads the project.

Image: Phytophthora root rot in young apple trees on M.793. Note the extensive root destruction in C compared to A.

Supplied by Sonja Coertze | Stellenbosch University Plant Disease Clinic.