Dee Twiddy, Mpho Matsetela and Dr Gerda Fourie
Forestry and Agricultural Biotechnology Institute, University of Pretoria
Little is currently known about the presence and prevalence of macadamia seedling diseases in commercial nurseries in South Africa. The impact of diseases on seedling production is also not well documented. A survey to document disease symptoms and identify causal agents from commercial nurseries was therefore conducted. Phytophthora and other oomycetes species were not included since Prof Adèlé McCloud from the University of Stellenbosch leads a research program focused on these species in macadamia orchards and nurseries.
The most common diseases across all surveyed nurseries were leaf diseases including brown leaf blight, yellow halo leaf spot, and Macadamia ringspot virus. A short overview of the causal agents, the typical symptoms and economic impact of these diseases is discussed below.
Leaf diseases
Yellow halo leaf spot
Caused by the fungal pathogens, Neopestalotiopsis clavispora and Pestalotiopsis versicolor, symptoms generally present as circular brown lesions or spots with a yellowing halo (Figure 1A). These causal agents were recently described from Australia and can impact both immature and mature leaves (Prasannath et al., 2020). This is also a common leaf disease on mature macadamia trees in orchards.
Brown leaf blight
Fungi from the Botryosphaeriaceae family are responsible for this disease which affects mature macadamia leaves in both nurseries and orchards (Li et al., 2022). Symptoms include brown discoloration, often with a dark margin, starting at the leaf tip and progressing down and inwards towards the petiole on mature macadamia leaves (Figure 1B). These light to dark brown lesions can result in Botryosphaeria stem dieback and tree death if the disease is allowed to progress.
Alternaria leaf blight
Also known as black leaf blight, disease symptoms include black lesions typically affecting young immature leaves i.e., the most recent flush or apical leaves of a seedling. This disease, caused by Alternaria fungi, can result in deformation and/or death of the seedling. Alternaria leaf blight is currently not common in South Africa. However, it is known to be present in Australian nurseries.
Other leaf blemishes
Leaf anthracnose or Colletotrichum leaf spot(Prasannath et al., 2020) was infrequently observed in surveyed nurseries. Symptoms are like other leaf blemishes but can be distinguished by irregular brown patches along the leaf edge (Figure 1C). Colletotrichum fungi are also known for causing husk rot in South Africa and have been detected in seedlings in Australian nurseries. Finally, circular mosaic yellowing, known as ringspot virus symptoms, were commonly observed in surveyed nurseries (Figure 1D). These are caused by an Orthotospovirus, a virus newly discovered by researchers at the Agricultural Research Council – Tropical and Subtropical Crops, in Mbombela. The virus is likely vectored through sap sucking insects, such as thrips.
Other diseases in nurseries
Stem and graft diebacks
The same Botryosphaeriaceae fungi that can cause brown leaf blight can cause stem dieback and tree death and are generally accompanied by dark staining of the vascular tissue. Points of entry are not restricted to the leaves for this disease as inadequate sanitation during grafting and wounds due to pruning as well as other injury or insect feeding damage, can all act as points of infection or triggers for these otherwise endophytic fungi. This can also be aggravated by overwatering.
Another disease of stems,Phomopsis graft dieback, was not observed in our survey but reported from Australia and may also be related to sanitation during grafting. In Phomopsis graft dieback, wilting leaves and dieback of the scion days after grafting are accompanied by a dark stain or discoloration at the graft union. These stem diseases highlight the need for good hygiene in nurseries especially during grafting, a high-risk process for pathogen infection.
Kernel rot and seed health
The condition of seed selected for germination can influence the potential for rotting before germination occurs. Current guidelines in Australia suggest mature nuts harvested in dry conditions only a few days after nut drop should be used to maximize the potential for uniform and high percentage germination. Bacterial kernel rot of macadamia is more likely to occur on nuts which lay on wet soil on the orchard floor for some time before being harvested. Bacterial kernel rot has a permeating foul odor and a wet patch around the seed in the sand bed. Although not a focus of our survey, this is of importance for improving nursery efficiency.
Disease management
Improving both the economic efficiency and quality of seedling production can be accomplished in large parts by good hygiene and management practices. Managing sources of contamination and infection pathways is critical. This includes movement of people through the nursery, grafting and pruning tools, pots, incoming plant material such as seeds and scions, and wounding which should be avoided especially in wet conditions. Adherence to sanitation practices, many already well established in the nursery industry, will aid in mitigating the economic impact of pathogens on the production of quality macadamia seedlings.
The impact of leaf diseases on the macadamia industry has not been quantified. However, the inevitable reduction of photosynthetic potential that leaf diseases represent will impact the speed of seedling development apart from progression to stem dieback. An increased load of seemingly endophytic fungi which could later be involved in diseases in established plantations is also a consideration for knock-on impact of diseases at a nursery level.
References
Li YP, Finnegan PM, Liu MJ, Zhao JX, Nie YL, and Tang L. 2023. First report of Neofusicoccum parvum causing leaf spot disease on Macadamia integrifolia in China. Plant Disease 107(2): 555.
Prasannath K, Galea VJ, Akinsanmi OA. 2020. Characterisation of leaf spots caused by Neopestalotiopsis clavispora and Colletotrichum siamense in macadamia in Australia. European Journal of Plant Pathology 156: 1219-1225.
A: Yellow halo leaf spot – characteristic yellow halo around brown spots.
B: Brown leaf blight – brown blotch starting from leaf tip progressing toward the petiole.
C: Anthracnose leaf spot – irregular brown spots along the leaf edge.
D: Ringspot virus – yellowing in a doughnut-like shape.
