Rust is caused by the fungal pathogen Tranzschelia discolor.
Common symptoms of the disease are twig cankers, leaf lesions and fruit lesions. Not all symptoms may develop in each growing season.
Twig cankers are the first symptoms in the spring. This cankers develop after petal fall in spring during fruit development on one year old wood. Symptoms are seen as blisters and longitudinal splits in the bark.
The infection starts with water- soaked lesions, which swell and rupture the epidermal tissue of the twig. Cankers are usually found on the upper, redish side of the twig. Few days afterwards (depending on the temperature) the cankers mature and produce rusty brown powdery masses of spezialised spores (urediniospores). This urediniospores are spiny and sharply constricted at the base. At the end of the saison old cankers could be still observed, they may persist in the following season but no longer viable spores are produced.
Leaf lesions develop usually after cankers form in spring and may continue till autumn. Defoliation can occur when high numbers of infections are on single leaves. First infected leaves are in close to the twig cankers (infection source). Lesions develop as pale yellowish green spots visible on both leaf surfaces. The lesions become bright yellow and angular and with age necrotic in the center. On lower leaf surfaces numerous spore pustules (uredinia) can bee found. They become rusty brown due to the production of powdery masses of urediniospores. At the end of the season leaf lesions my turn tark brown to black and they produce two- celled teliospores. These leaf lesions are angular shaped, small size and rusty brown.
Fruit lesions develop during growing season after the symptoms of the leaves. Firstly brownish spots with green halos on mature, yellow fruits are seen. When fruit redden, the halos become greenish- yellow. Numerous infections develop on each fruit and these can lead to secondary infections by other fungal pathogens like Monilinia, Colletotrichum, Alternaria or Cladosporium.
The fungal pathogen attacks plants of the genus Prunus, including almond, apricot, cherry, peach, nectarine, plum and prune. The fungus can be seperated by special forms, based on the host where it is found. These forms are T. discolor f. sp. persicae on peach, T. discolor f. sp. dulcis on almond, T. discolor f. sp. domesticae on prune.
The fungus has multiple spore stages, which develop on two different hosts (alternate hosts). The only alternate host which is reported from California is Anemone coronaria (Ranunculaceae). The different spore stages are urediniospores, teliospores, basidiospores and aeciospores. Only urediospores and teliospores are found on Prunus sp.
The single celled, rusty brown urediniospores are produced on peach and can re-infect peaches. This secondary infection and additionally spore production and reinfection causes epidemic damages on peach. The teliospores, which develope late in the season on peach are not able to reinfect peach. After overwintering, the teliospores germinate and produce basidiospores that infect the alternate host Anemone coronaria.
Aeciospores that are produced on A. coronaria infect only Prunus spp. and the infection produces the first cycle of urediniospores in the spring. A. coronaria is rare in stonefruit yards and probably not the source of first infection in the yards.
The fungus probably overwinters as mycelium in infected fruit wood from the previous summer or fall. In spring these infections become the twig cankers and that are the source of primary inoculum each year. Urediniospores from twig cankers infect leaves, where more spores are produced in lesions and under favourable conditions the disease becomes epidemic.
Adaskaveg JE, Soto-Estrada, A, Förster, H, Thompson, D, Hasey, J, Manji, BT, Teviotdale, B. (2000) Peach rust caused by Tranzschelia discolor in California. University of California. Agriculture and Natural Resources. http://anrcatalog.ucdavis.edu
Conditions for an infection - Output in FieldClimate.com
Urediniospores are dispersed by wind and rainfall. They germinate over a wide temperature range from 5°C to 30°C with an optimal temperature range of 10-25°C. The viability of inoculum and wetness are major factors for determination infection periods.
Leaf and twig infections can occurr over a wide range of wetness period (12 to 36hours) and temperatures (15 to 25°C). Under controlled conditions the optimal wetness duration and temperature for infection was 18 to 36 hours at 15°C to 20°C. The incubation period after infection is 8 to 10 days, whereas the incubation period for twig symptoms is 4 to 6 weeks at 20°C.