Phytophthora collar rot has caused this apple tree to collapse and die. There is a good chance that the disease will, or has already, spread to the adjacent trees. Replacing trees which have died from Phytophthora, especially in a high-density planting, is a nightmare, because the soil is contaminated, dead roots remain in the soil, and it is very difficult to manage young trees in an established mature block. It is almost impossible to eradicate the disease once it has invaded your high-density orchard. (Courtesy Bas van den Ende)

Collar rot and crown rot of apple trees and trunk rot of stone fruit trees (but rarely pear trees) are caused by the fungus phytophthora. They are among the most serious soil-borne diseases in the world. Diseased trees are mostly found where soil type, topography, climate and/or irrigation have contributed to long periods of saturated soil.

Phytophthora comes from the Greek for “plant destroyer.” Although there are several species of phytophthora that attack the roots of ornamental, fruit and forest trees, mainly two species of phytophthora attack deciduous fruit trees. Phytophthora cactorum rots the roots and trunks of apple trees, and Phytophthora cinnamomi rots the roots and trunks of stone fruit and avocado trees.

The phytophthora species are often referred to as water mold fungi, because they need free water to spread in soil and to infect orchards and nursery trees.

Although these different fungal species may vary somewhat in their temperature needs, tendency to attack root versus crown tissue, or aggressiveness against different rootstocks, all phytophthora species have two important characteristics in common: They thrive in wet soils, and they need free water to spread and cause significant disease.

Reproduction, spread and infection of trees by these fungi increases in saturated soil.

The pathogens can also move in contaminated soil on equipment, boots and vehicles; plant material; and water (irrigation and run-off).

In winter and for up to 10 years the fungus survives as thick-walled resting spores (zoospores) in the soil and rotted roots.

In spring, the zoospores germinate in wet warm soil and form sac-like sporangia that release many mobile zoospores into the soil water. The zoospores are attracted to tree roots and then infect the roots. The longer and more often soil is wet, the more likely the zoospores will reach, infect and kill vigorous and weak trees alike. However, young trees usually decline faster than mature trees.

Phytophthora cactorum is active from April to November (October to May in the Southern Hemisphere) in wet or saturated soil at 64 to 68 degrees Fahrenheit (18 to 20 degrees Celsius), and rootstocks are most susceptible from February to June (August to December).

So, if growers detect phytophthora in their fruit trees, the growers should actively eliminate the disease from October to December.

Roots need to breathe, just as we do, in a process called respiration, where the roots take in oxygen and release carbon dioxide.

Oxygen usually enters the soil in air from above and carbon dioxide moves out of the soil to the air above. If this flow of gases slows down or stops, such as in a soil that is saturated or slowly draining, toxic carbon dioxide builds up around the roots.

We cannot see this, but the drowning roots are now prime targets, for the phytophthora loves the saturated soil with plenty of food (i.e. dying roots) for the fungus.

It is often only when the trees try to grow in spring that we can see the damage. The roots cannot take up the water or nutrients needed for fast growth in spring. Ironically, the trees die from thirst, but the damage is due to drowned roots.

Collar rot and crown rot of apple trees

Phytophthora is among the most common causes of collar and crown rots of apple trees. The fungus rots the trunk at ground level (collar rot) or the main roots where they join the trunk (crown rot).

You can first see the disease in spring when trees are growing poorly, buds are developing slowly and leaves are chlorotic. During hot weather, the water-stressed trees start to wilt.

For collar and crown rots, the main symptoms appear when the bark is cut away. The margin between diseased and healthy tissue is sharply defined. The diseased tissue has zones of marbled tan to dark brown. Dead and diseased wood smells unpleasant and sour, hence the name sour sap. You can also see root rot when you dig up the soil around the base of the tree. The main and fibrous roots are dark brown and rotten.

Usually, trees slowly decline over several seasons and eventually die. Less often, trees suddenly collapse and die, especially after a very wet autumn or spring.

Trunk rot of stone fruit

Phytophthora trunk rot most often attacks peach and apricot trees, but sometimes also nectarine, plum and cherry trees.

Pale amber, cloudy drops of gum are exuded from the bark near the base of infected trees. The gum darkens with age until it is almost black, while new drops are exuded further up and around the trunk as the infection progresses.

When removed, the outer bark smells sickly-sweet and the inner bark and cambium in the lesion are discolored.

The old dead tissue is dry and uniformly rusty brown, whereas new dead tissue is sticky with gum and banded or mottled with shades of cream and brown.

The trunk is girdled and the tree dies, but, because the fungus grows faster along branches than around them, the lesion may extend 1 meter or more above ground level before girdling is complete.

Preventative measures

Prevention is much better than the cure, especially with phytophthora.

In many cases, phytophthora is a secondary disease. The prime cause is saturated soil, followed by root damage.

The cheapest and best methods for control of phytophthora are:

—Before the trees are planted, prepare the soil well with good surface drainage. Laser-grade the block with a slope of at least 1 in 80 along the traffic lanes so that excess water will drain from the surface of the soil under the trees.

—Careful management of soil and water.

Make good use of the surface soil taken from the traffic lanes by hilling (ridging) it up along the tree rows. This will increase the depth of good free-draining soil available to the tree roots. Hilling-up can increase the volume of surface soil in the tree row up to four times the volume without hilling-up. The surface soil should be deep, soft, stable, well-structured and well-drained.

Regardless of the method of irrigation, you must avoid too frequent or long periods of saturated soil, especially around the trees. In well-drained soil, zoospores of phytophthora are not released and cannot swim.

Fruit trees under hail net need different irrigation management than trees in the open. Soils dry more slowly under netting and trees can be watered less often. The net prevents water loss mainly because of lower evapotranspiration. Fruit trees under net with a 20 percent shading factor often need up to 20 percent less water. This means it is easy to over-water trees under netting and a greater risk of infecting the trees with phytophthora.

Chemical control

Phosphorus acid or Ridomil (mefenoxam) can help control phytophthora infections, especially in the early stages of the disease, but chemicals cannot resurrect trees that are badly damaged by phytophthora.

It is always best to protect your fruit trees by routinely spraying phosphorus acid (sold as Agri-fos) once in spring, summer and autumn. You must strictly follow the manufacturer’s recommendations for these chemicals, especially Ridomil.

Phosphorus acid sold as Agri-fos should not to be confused with phosphoric acid. Both chemicals are commonly used in horticulture, but they have very different uses. Phosphorus acid has some fungicidal activity and is often used to prevent or treat phytophthora root rot in a range of crops.

On the other hand, phosphoric acid is used as a phosphorus fertilizer, especially in hydroponic and fertigation systems, because it is very soluble in water. Each product has its specific use and they are not interchangeable. •

—by Bas van den Ende and Judy Tisdall

Bas van den Ende is a tree fruit consultant in Australia’s Goulburn Valley. Judy Tisdall is editor of the journal Soil & Tillage Research and senior lecturer in soil science at La Trobe University, Melbourne, Australia.