Specifics of the spread of rust and Stemphylium blight in pear orchards of Belarus

Introduction. The pear is a valuable crop, ranking second only to the apple in fruit production. The limiting factors for obtaining a high-quality harvest of fruits are its relatively low resistance to biotic and abiotic environmental factors, as well as infestation by a complex of pests, both indigenous and invasive.

Changes in cultivation technologies, such as the use of clonal rootstocks, foreign-bred cultivars, high-density planting, support structures, and drip irrigation, lead to higher yields. However, they also contribute to changes in the dominance structure of harmful organisms, and the introduction of planting material entails an increase in the virulence of phytopathogens that were previously of no economic significance, including brown spot and pear rust.

In Belarus, from 2008 to 2014, isolated signs of pear leaf rust infection were noted mainly in extensive orchards and private gardens. The disease is caused by fungi of the genus while is the dominant species, which is a heteroecious parasite with a two-year developmental cycle. The primary host plant is juniper, on which the fungus forms its sexual stage, and the intermediate host plant is the pear.

The first signs of rust on pear leaves appear in late April – early May in the form of small, rounded, orange-yellow spots 0.5 mm in diameter. Later, they increase in size, and the affected leaf tissue swells. During intense development of the disease, the fruit may also be affected.

Starting in 2015, the first epiphytotics of rust were noted in a number of European countries: in Central Russia, Ukraine, Latvia, Moldova, and Belarus. In 2016, staff at the laboratory for the protection of fruit crops of the RUE "Institute of Plant Protection" first observed the infection of foreign-bred pear cultivars with stemphyliosis in the republic's commercial orchards.

The disease is caused by the fungus , which affects:

• pear leaves in the form of small reddish-brown spots, which subsequently merge and form large necrotic areas;
• fruits in the form of rot, manifesting both during the fruit ripening period and during storage.

Throughout 2019–2021, the virulence of rust and stemphyliosis on pears increased, and epiphytotics became almost annual. Since detailed studies on the prevalence and development of diseases of the pear in the conditions of Belarus had not been conducted, this topic is relevant.

An assessment of the phytosanitary state of pear plantations was carried out in 2022–2023 by means of route surveys during the period from bud break to fruit ripening (May–September). The objects of study were commercial orchards and private plots in 5 regions of the republic:

• Vitebsk Region (RUE "Tolochin Cannery");
• Gomel Region (RUE "Gomel Regional Agricultural Experimental Station" of the NAS of Belarus);
• Minsk Region (SE "Voskhod" and RUE "Institute of Fruit Growing", RUE "Institute of Plant Protection" of the Minsk District, and SFH "Kletskiy" of the Kletsk District);
• Grodno Region (APC "Progress-Vertilishki");
• Mogilev Region (private plots only).

Observations on the development of stemphyliosis during fruit storage were carried out monthly in the fruit storage facilities of the RUE "Institute of Fruit Growing" and the RUE "Tolochin Cannery" throughout the entire storage period (up to 6 months).

Stationary observations on the development of pear rust were carried out in the experimental orchard of the RUE "Institute of Plant Protection", and on the development of stemphyliosis in the commercial orchard of the RUE "Tolochin Cannery".

Under laboratory conditions, microscope slides from spore traps were examined under a microscope, and the morphology of the spores was determined using software

Assessments of the prevalence and development of rust and stemphyliosis were carried out systematically throughout the entire growing season, at least every 14 days. To do this, 10 trees of the same cultivar were examined diagonally in a 10-hectare orchard. On each tree, 4 branches from 4 sides of the tree were inspected (25 leaves and fruits on each).

Disease development was determined in points according to the corresponding scales:

0 – No signs of infection;

1 – Small spots covering up to 1% of the leaf surface;

2 – Small or medium-sized spots covering 1–10% of the leaf surface;

3 – Large spots or numerous small spots covering 11–25% of the leaf surface;

4 – Large spots with dark fungal sporulation, covering 26–50% of the leaf surface;

5 – Spots covering more than 50% of the leaf surface.

0 – healthy fruit;

1 – rare spots on fruits;

2 – small spots on fruits;

3 – isolated spots (2–3) on fruits, up to 5 mm in diameter;

4 – significant number of large spots (5–10 mm), merging;

5 – numerous large spots (10 mm or more);

The prevalence of the disease was calculated using the formula: n

N, where P is the prevalence, the number of affected organs; N is the total number of evaluated organs.

The development of the disease was calculated using the formula:

100,

N×K, where R is the development of the disease, the sum of products of the number of affected organs multiplied by the corresponding severity score; N is the total number of evaluated organs; K is the highest severity score.

Results of the study and their discussion. As a result of route surveys of 7 commercial orchards in the Minsk, Gomel, Grodno, and Vitebsk regions in 2022–2023, it was established that in commercial pear plantings where intensive plant protection measures are carried out, rust is absent, while in private gardens the disease is noted everywhere, with the exception of the south-eastern regions of the Mogilev (Krasnopolye and Kostyukovichi districts) and Gomel (Rogachev district) regions.

Figure 1. – Prevalence of rust in pear plantings of the republic. (route surveys, 2022–2023)

Since the main host plant of pear rust is juniper, upon the onset of positive temperatures in the early spring period, surveys of decorative juniper plantings were conducted in the Minsk region (Ag. Priluki, Ag. Shchomyslitsa, Ag. Samokhvalovichi). As a result, it was established that the beginning of teliospore formation was noted at the end of April (24.04.2022 and 26.04.2023) at a minimum air temperature above 10.9 °C and relative humidity of air above 90.5%.

Figure 2. – Telial stage of fungi on juniper: a) on a young shoot; b) on skeletal branches

During the two-year observations, the infection of vegetative organs (shoots, skeletal branches, and leaves) of juniper was high, reaching 62.5–81.3%.

Figure 3. – Prevalence of rust in decorative juniper plantings, Minsk region.

As a result of microscopic analysis of the affected parts of the juniper, two species of teliospores were identified. According to the key for the identification of rust fungi by O. V. Mitrofanova, these species were determined as, and the fungus predominated in a 3:1 ratio.

Figure 4. – Teliospores of fungi of the genus, pathogens of pear rust (photo by V.V. Kalachev). Under the conditions of 2022, observations were carried out on the dynamics of rust development on pear in the orchards of the Minsk region: in the commercial orchard of the State Enterprise "Voskhod", the experimental orchard of the RUE "Institute of Plant Protection", and in private orchards. As a result, it was established that in the commercial orchard of the Minsk region, where minimal preventive measures were carried out (two treatments with a copper-containing fungicide Azofos 65% w.p.), the prevalence of rust on leaves did not exceed 38.0%, in the experimental orchard of the RUE "Institute of Plant Protection" without fungicide treatments in the current year, it amounted to 64.0–75.5%, and in private orchards reached 87%.

Figure 5 – Prevalence of rust in pear plantings

Observations of rust development on pear at the stationary site of the experimental orchard of the RUE "Institute of Plant Protection" showed that the first signs of leaf damage in 2022 were noted on 28.05, and in 2023 – on 01.06. The maximum air temperature in 2022 during this period was 21.4 °C, relative humidity of air – 99.9%, precipitation sum 12.3 mm, and in 2023 – 24.3 °C, 83.5%, and absence of precipitation, respectively.

Subsequently, under the favorable conditions of 2022 characterized by frequent precipitation, the development of the disease took the form of an explosive epiphytotic with a prevalence of up to 75.5%, while in the arid conditions of the first half of 2023, the development of rust was at a depressive-moderate level and ranged from 3.1% by the end of the first half of the growing season to 18.0% by the end of the growing season with a prevalence of 12.0–30%.

Thus, in commercial orchards where a full range of plant protection measures is carried out, no rust infection of pear was detected. In orchards where the number of fungicide treatments is minimal or absent, the prevalence of the disease reaches 38.1% and 87.0%, respectively.

The main source of infection for pear infection is juniper plants, the infection rate of which over the years of research was 62.5–83.1%.

Rust on pear in 2022, under favorable weather conditions, developed as an explosive epiphytotic; in the unfavorable conditions of 2023, the development was at a depressive-moderate level.

Surveys of pear in 2023 in commercial orchards during the growing season showed a widespread occurrence of stemphyliosis (brown spot) both in the form of leaf spotting and fruit rot.

Figure 6 – Symptoms of stemphyliosis on a pear leaf (a), on a fruit (b), and fungal spores (c)

Brown or stemphylial spotting was noted in mid-July on the leaves of all surveyed cultivars:

• of Belarusian breeding: Lagodnaya, Dukhmyanaya, and Prosto Maria;
• of foreign breeding: Patten, Nika, Yantarnaya, Conference.

The development of the disease in August ranged from depressive (2.6–2.9%) on Belarusian cultivars to moderate (10.0–17.3%) on foreign cultivars.

Figure 7 – Development of brown spot on leaves in intensive orchards (route surveys, 2022)

A survey of fruit storage facilities during the winter period showed that fruit infection with Stemphylium blight in the form of rot ranges from 3.0% in the Minsk region (RUE "Institute for Fruit Growing") to 4.5% in the Vitebsk region. The most susceptible cultivars were the winter-ripening cultivars Konferenciya, Nika, and Yantarnaya.

Thus, in commercial orchards, the harmfulness of Stemphylium blight on pear is increasing. The prevalence of the disease on leaves of cultivars of Belarusian breeding does not exceed 2.9%, while on leaves of foreign-bred cultivars it reaches 17.3%. Pear fruit infection during the storage period is also noted, with the prevalence of Stemphylium rot reaching 4.5%.

An intensive system of fungicide treatments fully protects commercial pear plantations from rust. In orchards with minimal fungicide application, the prevalence of the disease increases to 38.1%, and in cases of complete absence of treatments and in private plots, the disease is noted everywhere, affecting up to 87.0% of trees. The main source of infection for pear is juniper plantings, the infestation of which is 62.5–83.1%, and the species composition of rust pathogens is represented by fungi in a 3:1 ratio.

• Rust prevalence without protection — up to 87.0%
• Juniper infection rate — 62.5–83.1%
• Stemphylium blight on Belarusian cultivars — up to 2.9%
• Stemphylium blight on foreign cultivars — up to 17.3%
• Fruit infection during storage — 3.0–4.5%

Peculiarities of Stemphylium blight manifestation in the orchard and during storage

Stemphylium blight, or brown spot, remains a widespread pear disease in commercial orchards, however, its harmfulness is determined by cultivar resistance. On the leaf apparatus of cultivars of Belarusian breeding, disease development is maintained at a level of no more than 2.9%. At the same time, on leaves of foreign-bred cultivars, the infection reaches 17.3%. The pathogen causes additional losses during the storage period, when the prevalence of Stemphylium blight on fruits ranges from 3.0% to 4.5%.