Stone fruit disease research

Over the past century, especially since WW2, there has been a huge range of stone fruit disease research conducted by teams of pathologists and advisers from the Department of Agriculture, Waite Agricultural Research Institute, PIRSA and SARDI. Following are descriptions of some of the major disease research and extension programs that have been delivered to industry.

Apricot gummosis

Apricot gummosis (Eutypa armeniacae) was a devastating disease for orchardists causing significant loss of trees. Over a 3 year period in the mid 1950s, 15% of SA’s apricot trees were killed by gummosis. This resulted in a major research and extension program spanning more than 25 years.

An initial research team was established at Waite Agricultural Research Institute in 1947, comprising D.B. Adam (Reader Plant Pathology, Waite Ag Res Ins), J. Grace (Research Officer CSIRO) and N.T. Flentje (Plant Pathologist, Waite Ag Res Ins). The work of this initial team was continued through the 1950s and 1960s by Dr Maurice Carter (Waite Ag Res Ins) and Dr Bill Moller (SA Department of Agriculture) to assemble further information about Eutypa fungus as the cause of apricot gummosis and to develop control methods.

Key apricot gummosis research and extension activities over the years included:

• An initial article “Dieback of apricot trees in the Barossa Valley” was published by J.B. Harris in the SA Journal of Agriculture in 1932, followed by a further article in 1938 by D.B. Adam, “A progress report on a Gummosis (dieback) disease in South Australian apricot trees”.
• High wastage rates recorded in non irrigated apricot orchards. Initial gummosis study commences 1944.
• Apricot gummosis trial orchard planted at Nuriootpa High School 1945.
• Survey of gummosis incidence and pruning techniques 1953 and initial trial work on the property of Mr P.B. Boehm at Light Pass.
• Apricot gummosis results published by Plant Pathology Department, Waite Agricultural Research Institute. The research discovered disease is spread by spores on wind and pruning tools from infected dead wood. Pruning methods were identified as key way to control the disease (1955–56).
• Major program (May & June 1956) to locate and destroy dead apricot wood and derelict orchards, pruning education program to minimise cuts on main limbs, publicity drive via SA Journal of Agriculture (“Apricot Gummosis – a Warning” by R.L. Wishart March 1957), press, radio, leaflets and pruning demonstrations.
• In January 1957, Eutypa armeniacae, the cause of apricot gummosis was identified as the cause of “dying arm” in grapevines at Modbury, and confirmed that grapevines were associated with spread of apricot gummosis.
• Exhibit on pruning for gummosis control at Royal Show 1958.
• 16 mm film on apricot gummosis made 1960.
• Spore discharge timing studies show spores not released in June, July and early August (1963–64). Other common garden plant hosts identified.
• “Prune in June” campaign for apricot growers 1964–65.
• Evaluation of systemic fungicides in conjunction with pruning to reduce infection in 1960s and 1970s.

Crown gall

Crown gall is a common soil borne bacteria (Agrobacterium radiobacter var tumefaciens) that stimulates the growth of large highly debilitating root galls, especially in peach and almond trees. In the late 1960s, plant pathologist Dr Allen Kerr at the Waite Agricultural Research Institute (University of Adelaide) commenced studies into crown gall. He identified non pathogenic strains of Agrobacteria and proceeded to develop these into a root inoculant known as Agrocin 84. Field trials identified this inoculant when used on nursery trees achieved better than 95% field control of crown gall.

The Waite Agricultural Research Institute commenced supplying commercial stone fruit growers with the inoculant Agrocin 84 in 1973. It proved highly successful and is now applied routinely by nurserymen. Crown gall inoculant is now manufactured and supplied commercially throughout Australia, New Zealand, USA and South Africa.

Root knot nematode

Root knot nematode (Meloidogyne spp) is a frequent problem on sandy soils, causing stunting of trees and poor performance, especially in peaches. Initial treatments involved the use of soil fumigants and nematicides. However these normally only provide short term control of nematodes.

The development of root knot resistant rootstocks in the US was a major advance. The first of these was the seedling rootstock Nemaguard. Subsequently, a range of root knot resistant rootstocks have been introduced to Australia, including Okinawa, Flordaguard and Nemasun, and are used on peach, nectarine and apricot.

Today, rootstocks are used widely to deal with various orchard management issues including tree vigour, planting density, soil type as well as many soil pathogens.

Brown rot

From the late 1800s, orchardists had relied on copper and other protective fungicides to combat brown rot and a wide range of stone fruit fungal diseases. Work continued on use of these protectant fungicides at Berri and Blackwood Experimental Orchards over many years.

The launch of systemic fungicides in the late 1960s prompted new investigation into control of stone fruit fungal diseases. Research Officer Plant Pathology, Dr Peter Dry established a major project to investigate improved control measures for prune rust, freckle, brown rot and shot hole in apricots and peaches between 1969 and 1971. Brown Rot (Monilinia fructicola) was the most significant in apricots and peaches, causing severe blossom infections at flowering, and devastating brown rot fruit losses in the Waikerie and Golden Heights irrigation areas.

This project closely examined spray timing, an expanded range of protectant fungicides, and evaluated the new systemic fungicides like benomyl (these systemic fungicides would later be phased out due to the development of disease resistance). This fungal disease program also assessed the effectiveness of aerial spray application techniques.

Peter Dry’s Brown Rot research developed improved control strategies and focussed attention on the impact rainfall at flowering on infection levels. The gradual move from overhead to under tree irrigation greatly reduced the significance of brown rot.

Post harvest fungicide dips were also introduced in the late 1960s and early 1970s to prevent fruit rots developing during transit and marketing (Extension Bulletin 21/74, Post Harvest Control of Rot in Stone Fruits).

Blackheart

Blackheart (Verticillium spp) used to be a frequent soil borne disease, especially in apricots. This disease was frequently associated with waterlogging or inter-planting orchards with susceptible crops like tomato and potato. A major outbreak of Blackheart occurred following the very wet 1956 season. Improved orchard drainage and irrigation techniques have almost eliminated Blackheart as a disease problem.