Symptoms

• leaves on peach: small, pale-green to yellow, circular or irregular spots. Spots enlarge, and become darker to deep-purple, brown or black. The disease areas drop out, given a shot-hole appearance. A dark ring of disease tissue is left. Infected leaves turn yellow and drop off.

• leaves on plum fruit: symptoms may be different, large, sunken black lesions or only small pit-like lesions depending on the cultivar.

• twigs of peach: spring cankers occur on the overwintering twigs and on watersports before green shoots are produced; firstly small, water-soaked, dark blisters (1-10cm), sometimes girdle the twig, which cause the death of the top of the twig. Below the dead area (here are the bacteria present) a dark, so-called “black trip” occurs.

• twigs of plum, apricot: cankers are perennial and continue to develop in twigs of 2 and 3 years old.

Xanthomonas arboricola pv. pruni is a bacterial disease and is listed as a quarantine pest in the EEP A2 list. Mainly species of Prunus spp. are attacked by the bacterium and particularly in fruit crops such as almonds, peaches, cherries, plums and apricots. X. arboricola occurs now worldwide but was first found and described in North America (it is not really clear if it spread from there or has naturally a wide distribution range).

Biology

Xanthomonas arboricola is a aerobic, gram-negative bacterium.

The bacterium overwinters in the intercellular spaces of the cortex, phloem and xylem parenchyma of the peach tree. On plum and apricot summer cankers are formed in one season, which develops the following spring and provides a source of inoculum. Also, plum buds and fallen leaves are an overwintering source of bacterial disease.

In springtime the bacteria start to multiply and cause the epidermis to rupture- lesions are visible and are called spring canker. Inoculum from these cankers is disseminated by rain and wind and infects healthy plant tissues via stomata. On these leaves, lesions are developing, which exude bacteria and are called secondary infections.

Summer cankers develop in the green tissue of the shoot, but are sealed off by a periderm layer and dry out during summertime, which reduces the viability of the bacteria- therefore summer cankers in plum and peach are not of great importance as overwintering sites or initial infections the following season. In general, it is the late infections of shoots, occurring during rains just and during leaf fall in autumn which constitute the primary inoculum source for the following spring.

Modelling X. arboricola

Sensors: Air Temperature, Relative Humidity, Leafwetness and Precipitation
in FieldClimate.com we have three models of X. arboricola, depending on the development stage of the plant/infection of different plant material (blossom, leaf and fruit infection) and a propagation model.

The bacterial disease is favoured by warm, moderate seasons with temperatures of 10-28°C, light and frequently rainfalls with heavy winds and dews. Local dispersal is possible by rain splash in orchards.

Severity classes depend on the inoculum (last year’s epidemiology, the susceptibility of the variety and the weather conditions).

1. Modelling Blossom Infection
Temperatures between 15°C and 30°C, leaf wetness is more than 0.

In the graph a weak infection of the blossom has been determined, a moderate and severe infection was not calculated (has to reach 100% on the line) because the leaf wetness period was too short.

2. Modelling Fruit and Leaf Infection (additional precipitation is necessary to determine infections)
Temperatures between 15°C and 30°C, precipitation is more than 0, leaf wetness is more than 0, it is not night.

Also, this model separates between weak, moderate and severe infections in dependance on the sum of rain. . A weak infection (temp. between 15 and 30°C, precipitation and leaf wetness) was determined on the 16th of June because of the precipitation and the long leaf wetness period afterwards as well as the temperatures (10-17,5°C) during this time. A moderate infection (precipitation sum > 2mm) was not calculated (but nearly 98% of infection) as well as a severe infection (Precipitation sum > 5 mm) at that time.

3. Propagation Index
Temperatures between 15 and 30°C, leaf wetness more than 0 or relative humidity higher than 80%.