New wattle rust disease spreading rapidly
New pests and diseases in the spotlight at NCT Midlands field day
|Izette Greyling of the TPCP inspecting the rust damaged wattle compartment at Baynesfield.|
The latest destructive disease to hit South African forestry is a ‘rust’ that attacks black wattle, Acacia mearnsii. Tree farmers first detected it last year in the KwaZulu-Natal midlands. Since then, the TPCP team has been working around the clock to find out what it is, where it comes from, and how to stop it.
Izette Greyling of the TPCP was in KwaZulu-Natal recently to do some field work and gave a presentation to tree farmers who attended the NCT field day at Baynesfield, where the rust is already causing damage to young wattle compartments.
Izette says that the speed with which the rust is spreading is cause for concern. This last summer, it spread from Harding in KwaZulu-Natal to Mpumalanga. It is a rust fungus and causes defoliation in mild cases, and stem deformation in more severe cases.
The only good news around this disease, according to Izette, is that it has now been positively identified as belonging to the Uromycladium genus, and is thought to be of the fusisporum species, although this has still to be confirmed.
The TPCP is currently working with NCT and the ICFR to come up with management options, including selecting tolerant trees and doing chemical sprays on affected trees. The only short-term option at this stage is chemical control, and NCT is planning a series of trials in conjunction with the ICFR and TPCP.
“The rust is causing stress on the affected trees and there will be follow-up insect and pathogen attacks,” said Izette.
Some of the local farmers at the field day said they had noticed intensified mired infestations in compartments where the rust was present.
Another cause for concern for tree farmers, said Izette, is that the introduction of new pests and diseases attacking Eucalyptus species has accelerated in the past few years. This is consistent with the increasing volumes of cross border trade taking place around the world, allowing pests and pathogens to move around far from their host ranges.
She said that the three major pests in the KwaZulu-Natal midlands were Leptocybe, Thaumastocoris and Gonipterus.
New eucalyptus pests that have arrived in South Africa in the past two years include the red gum lerp psyllid (Glycaspis brimblecombeii) detected in 2012, and the shell lerp psyllid, in the genus Spondyliaspis, detected in 2013.
Both are sap-sucking insects that can cause a lot of damage. The search is on for potential biocontrol agents for these insects.
The biocontrol for Leptocybe (S. neseri) has been released in all major eucalyptus growing areas, and it is spreading on its own, which is good news as this pest has caused extensive damage to susceptible eucalyptus species, especially in Zululand.
“Leptocybe is getting worse – but that’s to be expected as it has had three years in which to reproduce. It will be another two years before the biocontrol can catch up,” said Yzette.
Meanwhile, there have been limited releases of the biocontrol agent for Thaumastocoris. According to Izette, the so-called ‘fairy fly’ is very small and difficult to rear in the lab, but evidence suggests it has survived in the field following its release.
Izette also said that there had been more severe outbreaks of old foes like Gonipterus, and the Cossid Moth, which is spreading, and has been reported in the Mooi River area recently.
After the indoor presentations, participants visited various sites on Baynesfield Estate. The 3 000ha Baynesfield estate is planted to wattle (just under 50%) and eucalyptus, mainly Smithii, a little macarthurii and now G x U since the Leptocybe infestations which ruled out the planting of G x C.
At one stop, we saw a young wattle compartment badly affected by the new rust disease. It causes loss of growth and forking and branching, with mortality of over 30% reported in some areas. The younger the trees affected, the higher the mortality rate. Some farmers are using general agricultural sprays to treat the rust, with good results.
Farmers could try and collect seed from resistant trees. Given the accelerating introduction of pests and diseases affecting forestry, it is becoming increasingly important for farmers to spread their risk by planting several different species on their farms.
|This Stihl earth augur, equipped with a long lasting, tungsten blade, produces uniform, high quality pits for planting. According to the operator, a pitting/planting team of five people can pit, water and plant 900 to 1 000 trees a day (six-hour shift) using the earth augur. The team needs to be multi-skilled with two or three team members taking turns operating the augur.|
|Craig Norris of NCT discussing the merits of planting G x U during the field day. Behind him is a compartment of 3.5 year-old G x U clones showing excellent growth at 1 100 metres altitude.|
*The TPCP has made a special request to tree farmers to report occurrences of the wattle rust to Jolanda Roux (firstname.lastname@example.org; 082 909 3202) or the diagnostic clinic (email@example.com). Early detection of the disease in new areas will greatly assist in assessing its status and informing management strategies.
*Published in June 2014