Huanglongbing not in South Africa but present in Africa

The vector of Huanglongbing (HLB) or Asian greening disease, the Asian citrus psyllid, Diaphorina citri, has been detected as far south in Africa as Tanzania and south-eastern Kenya, while the disease has already manifested itself in Ethiopia as well as Mauritius and Réunion. 

“We are fairly worried about the presence of the insect in Tanzania and Kenya. Last year we reviewed our initiatives against the disease. One of the cornerstones of the fight against Asian greening disease is planting disease-free plant material,” says Dr Paul Fourie of Citrus Research International (CRI). “For this reason we are busy with a cost-benefit analysis for South African nurseries to make recommendations on how to change over to insect-secure structures.”

South African nurseries are not yet insect-secure, but the experience of the Florida citrus industry has led the industries in California and Brazil to start the fight against the disease at propagation level.

Cederberg Tree Nursery in Citrusdal is leading the charge. “From a business perspective, we’re trying to be proactive. The threat of HLB is very serious and should it reach South Africa, we have to be in a position to keep it out of nurseries. We have invested a huge amount in our infrastructure,” says Scott McKenzie, manager at the nursery and horticulturalist.

The nursery’s newest greenhouses are equipped with two strategies that could be implemented within weeks if the disease reached the Western Cape, thereby making the greenhouses biosecure. The first strategy would be pressurisation fans that create positive pressure within the entrance cabins, keeping flying insects from entering through doorways. The other would be fine, insect-resistant net that covers any other potential entrance to the structure. The idea is that this net would allow air movement while still keeping out the insect vectors.

The nursery is constantly looking at new ways to maintain correct temperature and humidity within their greenhouses, for instance through the use of wet walls which enlarge the water surface area which water must pass by in order to move into the structure, thus cooling down the air. They propagated 530,000 trees of approximately 20 cultivars last year.

Through cultivation practices like growth medium (coir from Sri Lanka for young trees), fertiliser application and other systems changes like the sequence and timing of grafting and transplanting, Cederberg Tree Nursery has been able to reduce production times on young trees from 24 months to 16 to 18 months. Their three dominant rootstock cultivars are Carrizo, Rough Lemon and X639.

“The citrus nursery market is currently as strong as it has ever been. We have a waiting list up to 2019 for young trees, particularly late mandarin varieties, seedless lemons and some hard citrus like Valencias and navels,” says McKenzie. The nursery forms part of ANB Investments, the group that also includes the Citrogold variety development company, the Du Roi group of companies that provides subtropical and citrus plant material, as well as the ClemenGold brand.

“We want to start with a healthy tree. Biosecurity has gone a long way to stop the disease from further spreading in the USA,” he continues.

For more information:

Dr Paul Fourie 

Tel: +27 21 886 8612

Email: phfourie@sun.ac.za

Scott McKenzie

Tel: +27 21 880 7033

Email: scott@cederbergnursery.co.za