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WATTLE RUSTDISEASE

Craig Norris, NCT’s Forest Technology Services Manager

The wattle plantation industry is being affected by a serious new rust disease. Symptoms were first noticed in December 2012 in the Eston district. During the summer months of 2013/14, the rust spread throughout the wattle growing areas in southern KZN, Melmoth district and into northern KZN.

The pathogen has been identified by the Tree Protection Co-operative Programme (TPCP) as an Uromycladium spp. These fungi have a complex life cycle which can involve the formation of up to three types of fruiting bodies and four spore types. In all species, initial infection occurs when spores germinate on a leaf or branch and penetrate the surface, causing the formation of a spot of diseased tissue on the host (figure 1).

The fungus can produce vegetative “repeating” stages (urediniospores). These spores are readily wind-dispersed to other plants where germination, infection, and further production may occur. Thus several crops of spores may be produced during the growing season. Later in the growing season fruiting bodies produce wind-dispersed spores, which can overwinter.

Figure 1: Spots of diseased tissue caused by infection of the rust.

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The rust appears to need moist, low light conditions to infect trees. This means that spring and early summer months are suited for the spread of the disease. Prolonged spells of overcast wet weather result in the appearance of a slime like substance on branches and leaves (figure 2).

Figure 2: Slime like substance forming on leaves and branches after wet conditions.

The pathogen infects trees of all age classes and results in trees dropping their leaves and die back of growing shoots. Infested compartments are likely to require corrective pruning operations, additional weed control (light entering through canopy) and there will be a loss in growth (figure 3). In severely affected young compartments mortalities of up to 30% have been recorded.

Figure 3: Top die-back and leaf-drop results in weed infestations and poor tree form.

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A wattle rust steering committee has been formed to identify and support various research initiatives. These projects can be grouped into five key areas:

Wattle rust identification, biology, epidemiologyThis work will confirm the identity of the rust and describe the life cycle; including patterns, causes and

effects of the disease.

Chemical controlEmergency registration for three chemicals has been requested to allow for immediate chemical control.

In addition, a series of trials will be established to test various other chemicals and to register the most effective for use in forestry. This work will also result in the development of best operating procedures for the use of these chemicals.

Monitoring & risk predictionA remote sensing based system will be developed for detecting and monitoring wattle rust. This system

will be useful in helping to anticipate areas likely to be affected and managing infestations.

Impact assessment and quantificationThis work will allow for a better understanding of the growth impacts of the disease on Wattle and also the

relationship between the rust and insect infestations.

Tree breedingThe wattle breeding programme at the ICFR will focus on selecting for tolerance to the rust with the

aim of establishing seed orchards that can produce more resistant material. In addition, material will be collected from trees that appear to be more rust tolerant for propagation via cuttings. The possibility of producing tolerant material by hybridising black and green wattle will also be investigated.

It is anticipated that this rust will spread through the mail wattle growing areas during the coming summer months and poses a serious risk to the Industry. However, with the help of research partners, the industry has mobilised resources to develop an integrated pest management strategy to deal with the threat.

Figure 4: Green wattle (foreground) appears to be more tolerant than black wattle (background).