Pear rust: peculiarities of pathogen biology measures for control and profilactic
Authors: Cherniy A. 
Published: November 2019

Goal. To carry out an analysis of the spread, peculiarities of the biology of the rust pathogen — the fungus Gymnosporangium sabinae (Dicks.) Wint., the dynamics of the disease, control measures.

Methods of investigation. Retrospective analysis of pear rust spread for the period 1960—2010; information and analytical analysis of the peculiarities of biology and the life cycle of the pathogen; field observations of the dynamics of disease development on host plants in 2015—2019; analytical generalization of control measures taking into account the biology of the pathogen.

Results. The periods of disease spread in different regions for 1960 — 2019 were established. The main areas of rust spread and harmfulness are: the southern coastal and foothill zone of Crimea, the Black Sea zones of the Krasnodar and Stavropol regions, and the Western Georgia. Between 1975 and 2010, the disease did not spread and did not demonstrate itself in other regions. Since 2012, rust has been spreading and accelerating development in Ukraine, Belarus, the Non-Black Soil zone of Russia. In 2016 — 2018, especially in 2019, mass infection of hear was detected — there was a threatening situation of epiphytoty on a large territory. The pear rust is caused by the highly specialized fungus Gymnosporangium sabinae Wint., which develops on two host plants. The main host of the rust pathogen is juniper (Juniperus sp.), the intermediate host is pear (Pyrus sp). The basic information of biology is summarized and disease cycle of the pathogen is presented. The peculiarity of the pathogen biology is an incomplete cycle of development, which consists of two stages: aecio-stage (pear) and telio-stage (juniper), which results in 4 types of spores. The disease cycle lasts almost two years and consists of two consecutive processes: 1 — formation of basidiospores on juniper and their distribution; 2 — germination of basidiospores and formation of aeciospores on pear. Basidiospores are dispersed by wind in the radius of 40—50 km and infects pear in the spring, aeciospores infects juniper in the autumn. The development of the fungus occurs in a wide temperature range from 3 to 30°C (optimum 18°C) and relative humidity of 85%. On pear rust develops over 4—5 months (April — September). The dynamics of disease development depends on the sporulatuion rate of basidiospores on the juniper and their spread to the pear; formation of aecia and ripening of aeciospores on pears. During the growing season, depending on the weather and climatic conditions, there are 4—5 periods of sporulation, which are the most threatening for pear infection. Symptoms of the disease are very clear. In the and late April, small yellowish-green spots of irregular shape with a diameter of about 0.5 cm appear on the upper side of the leaf. They gradually expand, become red or red-orange, affect the tissue and inflate. In the course of the development of the disease from the underside of the leaf on the same spots appear well-visible conical or soy-shaped outgrowths, arranged in groups. They are red-brown or rusty. Infected shoots become thick and short, and severely affected shoots dry up; the fruits grow slower and became deformed. Disease results in a complex of negative effects: causes increased evaporation, premature drying and fall of leaves; in plants, photosynthesis is getting worse and metabolism is impaired. The strong development of the disease leads to the loss of winter hardiness of trees and their death. Protection and prevention measures include sanitary practices aimed at reducing the rust infection, use of disease-resistant varieties, and chemical treatments. Sanitary and organizational measures: in spring — cutting of severely affected shoots and skeletal branches, cleaning of wounds with subsequent disinfection; whitewa­shing of trunks and skeletal branches with a solution of fresh lime with the addition of copper-containing preparations. Collect and burn leaves after the fall; treat the trees with a 7% urea solution. Dig stem circles and treat the soil surface with a 5% solution of copper sulfate. Juniper bushes, severely affected by the disease, dig in and remove. To reduce the risk of tree disease, it is advisable to plant resistant to rust pear cultivars. Curative measures: spraying with copper and sulfur-containing fungicides. Important: the timing and feasibility of chemical treatments in rust control are they are conditioned by periods of basidiospore formation and dispersion and the weather conditions. The application performs in the green cone stage, taking into account 3—4 hours of rainfall during this period and temperature not lower than 9°C. At “white bud” stage and after the fall of 75% of the petals, a rain lasting at least two hours is required. The delay of rainfall shifts the application timing. The following two treatments are carried out during the period of fruit growth, taking into account that young leaves are the most susceptible to disease. For spraying during these periods, one of the following fungicides may be recommended: Cuproksate, 34.5% EC, Kuprosil, 10% SC, Champion, 77% WP, Blue bordo, 77% WG, copper chloride, 90% WP, colloidal sulfur, 77% p or its substitutes (Cumulus DF, 80% WG, Tiowit Jet, 80% WG, Poliram DF, 70% WG. The use of pesticides should be alternated to avoid the formation of resistance.

Conclusions. Rust is an extremely dangerous disease that affects all aboveground pear organs and outweighs the damage of scab and cancer taken together. In 2016—2018, especially in 2019 — the disease has become widespread and pear infection had character of epiphytoty. Pear rust is caused by the highly specialized fungus Gymnosporangium sabinae (Dicks.) Wint. — two-host pathogen; the main host of rust pathogen — juniper, intermediate — pear. A characteristic feature of the biology of the rust pathogen is the incomplete disease cycle, which consists of 2 stages resulting in the formation of spores: a telio-stage on juniper (formation of basidiospores) and aecio-stage (formation of aeciospores) on pear. Spores infect juniper and pear in a certain sequence. The disease cycle goes in a circle: the development of basidiospores on juniper and their dispersal on pear; the development of aeciospores on pear and their dispersion on juniper. During this cycle, neither juniper nor pear can infect themselves. Infection begins in spring with the development of basidia and ripening of the basidiospores and their dispersion by wind in a radius of 40—50 km to different organs of the pear. Pear rust develops for 4—5 months (April — September), begins with the penetration of growth tubes and germination of basidiospores in the tissue of the leaf. The dynamics of the disease development depends on the rate of sporulation of basidiospores on juniper and their spread to pear. For effective protection of pear gardens from rust it is necessary to apply a set of measures aimed at preventing of infection. Sanitary and organizational measures: in spring — cutting of severely affected shoots and skeletal branches, cleaning of wounds with subsequent disinfection; whitewashing of trunks and skeletal branches with a solution of fresh lime with the addition of copper-containing preparations. Curative measures — carrying out chemical treatments of copper and sulfur-containing fungicides. It is important to adjust the timing and feasibility of chemical treatments with the periods of formation and dispersion of basidiospores and the weather conditions of the growing season.