Viral Economics

1. Direct losses

Viral infections can result in direct losses when trees die, or fruit is affected. For example, apple chlorotic leaf spot virus, apple stem grooving virus, apple stem pitting virus, prune dwarf virus, and Prunus necrotic ringspot virus can all cause graft union failure in susceptible cultivars.

Prunus necrotic ringspot and prune dwarf viruses can also decrease bud take after grafting. Infection of either scion or rootstock with prune dwarf virus has been reported to reduce bud take by up to 50% in almonds. A German study found that viral infections could reduce bud take in apple nurseries by as much as two-thirds.

Pear stony pit caused by apple stem pitting virus is an example of fruit damage related to viral infection. Affected pears are deformed and gritty. Up to 94% of the fruit on an infected tree can be damaged in a bad year.

In SA, plum viroid 1 causes sporadic losses in certain plum cultivars due to marbling. Infected trees look fine but abnormal appearance and quality render some fruit unmarketable. The occurrence of marbling seems to be influenced by environmental stress – a pattern also seen with many viral infections.

2. Reduced growth and yields

Failure to thrive is arguably a more significant – and underrated – outcome of viral infections in deciduous fruit trees than direct losses. One reason is that poor tree performance due to infection can be hard to disentangle from other causes such as drought or replant disease. Another is that the consequences of infection are cultivar dependent.

For example, researchers reported yield reductions of 40 – 50% in Golden Delicious and Red Delicious infected with apple mosaic virus, while yield reductions in McIntosh were less than 10%.

Infections with multiple viruses further muddy the waters. Whereas peach trees infected with Prunus necrotic ringspot virus had yield reductions of 18%, those infected with both Prunus necrotic ringspot and prune dwarf virus had reductions of 30%. Other authors have noted up to 60% lower yields in peaches simultaneously infected with both these viruses.

Many studies have documented poor growth associated with viral infections. German apple trees infected with apple mosaic virus grew 40% less than uninfected trees over five years. A New Zealand trial found that Freyberg apple trees infected with apple mosaic virus produced 42% less wood over 12 years than uninfected trees. Golden Delicious is a parent of Freyberg.

A New Zealand trial showed that Jonathan apples were smaller when trees showed moderate to severe signs of mosaic compared to fruit from trees with mild or no signs.

Viral infections have similar potential to stunt growth in nursery trees. For example, experimental infections of nursery trees with apple stem grooving virus reduced height by an average of 23% and stem diameter by an average of 14%, compared to uninfected controls. Golden Delicious was the worst affected cultivar in the trial, with a 64% reduction in height and a 43% reduction in stem diameter.

3. Reduced survival

Lack of performance may shorten the lifespan of a tree when the grower loses patience with it. But the viral infections themselves can also reduce tree survival. For example, up to 80% of trees died in one study on four peach cultivars infected with both Prunus necrotic ringspot and prune dwarf virus.

Another example of reduced survival is top-working disease – the decline of apple trees within a few years of grafting. Apple chlorotic leaf spot, stem grooving and stem pitting viruses were reported to cause top-working disease in apple trees in Japan and India. It tends to affect only some rootstocks – trees grafted on Malling and Malling-Merton rootstocks do not develop the disease.

4. Management costs

Infected trees can often appear healthy provided they are not stressed in any way. This suggests that the impact of viruses can be mitigated through careful management. But ensuring perfect fertilisation and irrigation at all times comes at a cost – and might be impossible during times of drought.

Furthermore, infected trees may not be able to carry the same crop load as uninfected trees. Meanwhile, they are a potential source of infection for healthy trees. So the best strategy is to keep viruses away by only planting trees that have tested free.

In SA, plant material should be tested free of apple chlorotic leaf spot virus, apple mosaic virus, apple stem grooving virus, apple stem pitting virus, prune dwarf virus, Prunus necrotic ringspot virus, and plum viroid 1. These are the viruses and viroid included in the Deciduous Fruit Plant Improvement Scheme.

Testing and certification of plant material is an unavoidable cost in a virus-ridden world. There is no doubt, however, that this form of prevention is far more economical than trying to manage chronically diseased trees.

5. Quarantine

Viruses affect the movement of plant material. Even though several viruses of pome- and stone-fruit trees already occur in SA, there are many others that we have so far escaped. Plum pox virus is a good example.
Plum pox has been described as one of the most destructive diseases of stone fruit. Unlike the viruses listed above, plum pox virus is transmitted not only by grafting but also by aphids. One study published in 2006 estimated the global cost of plum pox management over 30 years at more than €10 billion.

Plum pox renders fruit unmarketable. Because plum pox virus is not yet ubiquitous, it is a quarantine disease in certain countries, thereby affecting trade.

Diseases such as plum pox are one reason for the strict regulation of plant-material importation. Although our deciduous-fruit industry clearly has to be protected from invasive pests and diseases, this does constitute yet another cost of viruses.

Many of the figures for reduced tree performance due to viral infections are decades old, such as a South African publication from 1949 stating that apple yields were reduced by a third in trees infected with apple mosaic virus.

More recent reports are scarce because testing and certification of plant material have become standard practice in most countries – sparing many growers losses due to viral infections. The risk is that we begin to focus so much on the price of control that we forget the much higher price of disease.