The diagnostic services of the Plant Protection Division at the Agricultural Research Council (ARC), Infruitec-Nietvoorbij in Stellenbosch has confirmed the presence of stem canker pathogens in all areas where pome and stone fruit is produced.
These telltale symptoms of stem cankers include dieback of shoots, cankers on shoots with fungal gummosis (shoot and trunk diseases) and fruit rot. Stem cankers are caused by fungal pathogen infections of species from the genera Diaporthaceae and Leucostomaceae. There is no cure for fungal stem cankers, the survival of which depends on environmental conditions. Warm, humid conditions in spring and summer are favourable for infection and lesion development. Symptoms can vary depending on the plant pathogen, inoculum pressure, the growth stage and age of trees, cultivar susceptibility, and abnormal weather conditions. Canker pathogens survive from one year to the next in cankers on the trees, or on infected, pruned branches and twigs left in the orchard. Infection of healthy trees occurs mainly through mechanical injuries such as pruning wounds from tools disseminating the spores from one tree to another. Infection may also occur through buds, bud scale scars, stipule scars, fruit scars, blossoms or young shoots. Canker infection sites turn into reddish-brown, sunken, pointed lesions with marginal longitudinal cracks. Small amounts of amber-coloured gum produced in and around the infection site may occur (Fig. 1). The cankers are more or less elliptical in shape, because the lesions extend more rapidly up and down the length of the branch. As the lesions get older, the affected bark at the edges turns up and peels off. These infection sites are well known on pome and stone fruit trees.
Prevention of stem canker infection
Because stem cankers are incurable, putting some of these preventative measures in place – where possible – is critical:
1. Ensure deep, well-drained soil and good air drainage to minimise infection, when establishing new orchards.
2. Use certified disease-free nursery plant material when establishing new orchards.
3. Avoid new plantings on the downwind side of heavily infected orchards.
4. Disinfect pruning tools between cuts with Ethanol (75%) or bleach solution (50%).
5. Use pruning wound sealant on all wounds wider than 20 mm.
6. Cut out all dead wood and infected shoots towards the end of winter and the beginning of spring, as wounds heal quicker during this period.
7. Avoid summer pruning because available inoculum can germinate and colonise wounds rapidly at this time.
8. Remove the pruned wood and any fruit mummies from the orchard, and burn them.
9. Avoid additional tree stress such as nutritional deficiencies and water stress during the hot summer months.
10. Prevent winter injury, sunburn and insect damage and promote optimum plant health, which facilitate rapid wound healing.
Diagnostic samples of apple, peach, pear and plum cultivars and their rootstocks taken over 23 years, showed the presence of fungal stem canker pathogens such as Diaporthe and Leucostoma spp. These samples were taken in all the growing areas of the Western Cape and in some of the areas of the eastern Free State, Vaalharts and Mpumalanga (See Table 1 and Fig 2).
Shoots with stem cankers can be submitted to the Plant Protection Division at ARC Infruitec-Nietvoorbij in Stellenbosch for diagnostic analysis.