Plum rust

The fungus is heteroecious, meaning it requires two unrelated hosts to complete its life cycle. It often uses anemone species as primary hosts before spreading to stone fruit trees.

The pathogen produces several types of spores, including aeciospores, urediniospores, and teliospores, which facilitate infection and long-term survival in the orchard environment.

Spore dissemination occurs primarily via wind and rain splashes. Once they land on susceptible leaf surfaces, they germinate in the presence of moisture and enter the plant tissues through stomata.

The fungus overwinters as teliospores on fallen leaves or as mycelium within the buds and shoots of the infected tree, allowing it to re-emerge every spring.

Plum rust affects various stone fruits within the Rosaceae family, including plums, apricots, peaches, nectarines, and occasionally cherries or almonds.

The disease primarily targets the foliage. Severe infections lead to the premature shedding of leaves, which significantly weakens the trees and reduces their photosynthetic capacity.

Because leaves fall early in the season, the trees cannot accumulate sufficient carbohydrate reserves, leading to poor winter hardiness and increased susceptibility to frost damage.

In subsequent years, trees weakened by rust exhibit stunted growth, reduced fruit yield, and smaller, lower-quality produce that ripens unevenly.

Persistent infection over several years can result in branch dieback and overall decline of the orchard, significantly impacting economic output for fruit growers.

Initial symptoms typically appear in mid-summer, often following periods of high rainfall or increased humidity, which are ideal for spore germination.

The development of the fungus is highly dependent on environmental conditions, with optimal temperatures for infection ranging between 18°C and 25°C.

Late summer and early autumn represent the period of maximum disease spread, as the production of urediniospores accelerates during this time, leading to rapid secondary infections.

The incubation period usually lasts between 10 to 14 days, depending on weather conditions. High humidity significantly shortens the time from infection to visible symptom expression.

The disease cycle concludes as autumn progresses, with the fungus shifting into a dormant stage on leaf litter until the next growing season begins.

Early symptoms include the appearance of small, yellow or light green chlorotic spots on the upper surface of the leaves, which gradually expand over time.

On the underside of the leaves, corresponding to the upper spots, characteristic rusty-brown pustules develop, packed with fungal spores that resemble fine powder.

As the infection progresses, the leaves may curl, dry out, and drop prematurely, often leaving the branches sparsely foliated by late summer.

• Yellowish spotting on the leaf upper side.
• Rusty brown, powdery spore masses on the leaf undersides.
• Premature defoliation starting in August.
• Reduced vigor and stunted branch development.

The disease is easily distinguished from other leaf spots by the distinct rusty, powdery texture of the pustules found exclusively on the lower leaf surface.

The most critical sanitary measure is the thorough removal and destruction of all fallen leaves, as they harbor the spores that sustain the infection cycle.

Pruning trees to improve air circulation and sunlight penetration is essential, as this helps dry foliage quickly and creates an unfavorable environment for fungal development.

Chemical control involving fungicides should be applied, particularly after flowering, to protect the new foliage from initial spore colonization throughout the season.

Copper-based fungicides are often used early in the season, while systemic fungicides (such as triazoles or strobilurins) provide effective protection during the peak summer months.

Maintain tree health through balanced fertilization and proper irrigation to bolster the plant's natural immune system against fungal pathogen attacks.