The Plant Disease Clinic at Stellenbosch University – by Anna Mouton

We are all familiar with specialists in human medicine — everything from neurologists to urologists. But did you know of the specialists that offer diagnostics for your sick plants?

Just as most people rarely need a medical specialist, most growers might seldom need a plant pathologist. But when you do need it, the service is critical, explains Stephen Rabe, horticultural consultant, and chair of the Hortgro Science Advisory Council. “When you make use of a specialist, it’s for a very important solution to a particular problem,” says Rabe. “The Plant Disease Clinic is not front of mind to your average producer. But if we don’t have these specialists that can diagnose a problem, it’s going to be a big loss to growers.”

All sorts of samples

The Plant Disease Clinic was established in 2000 in response to a growing number of people turning up at the Department of Plant Pathology with samples of diseased plants. Industry bodies such as Hortgro supported the initial development of the Clinic. Today it is funded through income generated from services.

Sonja Coertze manages the Plant Disease Clinic as part of her duties at the Department of Plant Pathology. She explains that the Clinic and the Department are closely knit. “Over the years we’ve made observations — for example, increases in known diseases or detections of new pathogens — that are eventually taken further by students as research projects.”

The Clinic employs three full-time staff. Brenda de Wee has been involved in the Clinic from the start and is responsible for hands-on laboratory functions such as media preparation and sample processing. Diagnostic examinations and pathogen isolations are performed by Lonette Smit and Doré de Villiers, who both hold postgraduate degrees in plant pathology.

“We prefer to work with plant samples because that is our field,” says Coertze, “but we basically cover everything.” In addition to plant material, the Plant Disease Clinic receives soil, water, insects, swabs, cultures — even occasionally food.

Nearly half of all samples are submitted by the fruit industry, including pome and stone fruit, wine and table grapes, citrus, and berries. About a fifth are vegetables, including onions, tomatoes, potatoes, peppers, and pumpkins. The rest of the samples are a mix of everything from cereal crops to ornamental plants.

Samples are received from all over South Africa, as well as from countries such as Namibia, Zambia, and Mozambique. Samples from other countries must be accompanied by import permits from DALRRD. “Our lives are definitely not boring,” asserts Coertze. “You never know what patients to expect!”

Test, don’t guess

“In many cases, we see a disease or a problem on a tree, or trees that die, and we don’t know the exact cause,” says Nico Ferreira, technical advisor at Fruitmax Agri. “And then I can’t tell a grower how to fix the problem. So I prefer to send samples to the Clinic so that I know what the problem is, and we can find a solution.”

“Symptoms on a plant are just an expression of a problem,” says Coertze. “Unfortunately, there are many cases where a diagnosis cannot be based on symptoms, and correct diagnosis affects management strategies. That’s why we always say, test, don’t guess.”

When plant material arrives, it is scrutinised for abnormalities. Bagged plants are removed from the soil and their roots washed. Everything is cut up to ensure that no internal symptoms are missed — the plant is effectively given a post-mortem examination. “Many of the people who send us samples know their crops and the diseases that affect those crops,” says Coertze. “So the samples they send us often have atypical symptoms. We can’t be certain whether it’s a fungal or bacterial disease, so we do a general analysis.”

Isolation and identification of bacteria and fungi are important components of disease diagnostics, requiring both knowledge and experience. The pathologist must be able to spot disease-causing organisms among contaminants on artificial growth media and must judge which are the most likely primary cause of a disease problem.

Fungi can also be coaxed to reveal themselves by incubating samples in humidity chambers. Rosellinia will produce characteristic pear-shaped mycelia from apple-tree roots placed in the chambers, while organisms causing downy mildew and late blight will develop characteristic spore-bearing structures.

When oomycetes such as Phytophthora are suspected, the pathologists have yet another trick up their sleeves — root-baiting. Sections of diseased roots are placed in water and incubated to allow the oomycetes to release swimming spores. The pathologists float young citrus leaves on the surface of the water, which acts as bait for the swimming spores. Infected leaves can be transferred to selective media to grow the oomycete for identification.

The Plant Disease Clinic also offers general genome-based identification of the disease-causing organisms that they isolate.

Sending plants to the Clinic

For plants as for people, the first step in making a diagnosis is taking a thorough history. Coertze stresses the importance of sending a completed submission form with each sample. Forms can also be emailed. The forms are available on the Plant Disease Clinic website.

“Clients should preferably send us three plants. We handle that as a single sample submission,” says Coertze. She explains that the pathologists need to analyse more than one lesion from more than one plant sample with similar symptoms to ensure an accurate diagnosis.

Plant material should be packed in a sturdy box with ice bricks. Add newspaper or other padding to protect the contents from bumps and bruises. And remember, no sample ever gets fresher, so use a courier or deliver in person. For the same reason, rather send samples early in the week than risk them arriving at the Clinic too late for processing, or worse, spending the weekend in a depot somewhere.

Coertze cautions against sending plant material after control measures have already been applied. “It happens fairly often that we receive a sample, and we culture nothing at all from it. Then we find out that the grower sprayed before sending. If you want to spray, fine, but first collect your sample.”

In most cases, clients can expect a result within ten working days after the sample arrives at the Clinic. Find out more on the Plant Disease Clinic website, or contact them at 021 808 4223 or sc2@sun.ac.za.

Caption. From left to right: Doré de Villiers, Lonette Smit, Sonja Coertze, Brenda de Wee.