STONE FRUIT - VIRUSES

Casual organism:
Taphrina deformans (berk.) tul.
Taphrina pruni (fuck.) tul.
Host range: peach, almond, plum, ornamental plums.
Occurrence and importance: These funguses cause serious damage in plantations and tree nurseries in colder locations, and in cooler, wetter climate. The extent of the caused damage varies every year and according to species susceptibility. Significantly susceptible species include: Elberta, J. H. Hale, Harko, Nektared 4.
For infection in early spring a 5°C average temperature is suitable while for optimal multiplication 5-10 °C is needed accompanied by rain, fog and dew. The pathogen overwinters on the tree bark, the branches and the fallen leaves with Ascuses (sexual reproduction formula of T. defomans and T.pruni). If these fungi enter the soil they can remain viable as saprophytes for years. The spores developing in the ascuses multiply via sprouting before scattering. The mycelium of the fungus spreads and feeds in the channels between the cells.

Their size becomes bigger, the leaves thicken and get deformed. Even the flowers can be damaged. They may get deformed and turn infertile. Fruits may also be infected, with the skin of the fruit developing blisters before the infected spots turn brown and rot. Depending on the intensity of the illness the tree may lose its entire crop.

Control: It is very important to choose a suitable site for the orchards, as low-lying areas are preferred by the pathogen. Proper pruning is an effective method of control. As the susceptibility of species varies, the intensity of treatment should be adapted accordingly. Spraying with lime sulphur and chemicals containing copper at leaf fall and at the end of winter will also reduce the risk of infection. After budburst, contact and systemic chemicals for continuous control should be used, especially under weather conditions that favour infection. To prevent the development of resistance, alternate chemicals.

Sphaerotheca pannosa var. persicae
Sphaerotheca pannosa var. rosae

Host range: peach, almond, rose hip.
Occurrence and importance: Peach powdery mildew is an important pathogen of peach trees, with no known totally resistant species, that affects trees in nurseries and yielding plantations every year to a variable extent. The importance of this pathogen is increasing with the increasing use of more susceptible nectarine species. Peach powdery mildew affects yields, but in particular quality in peach production. Wild peach species are more susceptible.

Symptoms: Young leaves, shots and fruit are equally susceptible to infection. The first symptoms may occur at the end of petal fall on the young leaves that appear on the twigs and on the leaves of the forming sprouts. The shoots may become totally bare over time and wither at their apex. The infected plant become covered in a floury, felty layer of greyish-white fungal mycelium. The white spots on the infected fruits are not too striking but in the growing period these spots turn greyish, then enlarged, brown and corky. In susceptible species considerable foliage and fruit fall may occur.

Disease cycle: The pathogen overwinters in the buds. The overwintering mycelium can be found on the inner bud scales. Actual infection takes place only after foliation. Conidyum (asexual reproductive formula) chains form on the developing plant parts that germinate between 20-25°C after a one- or two-week incubation period. The Cleistoteciums (sexual reproduction formula of the fungus which has role in spreading of the pest) do not play any role in the overwintering of the pathogenic.

Control: The majority of the overwintering mycelia can be removed from the trees by professional, traditional pruning since this involves removal of the upper third of each shoot. For twig pruning, selection and removal of infected twigs is used. Chemical control is predominantly based on prevention using sulphur compounds. When symptoms appear, organic and chiefly absorbing chemicals with curative effect can prove to be efficient such as azols.

  

Host range: apricot, peach, cherry, almond, sour cherry, strawberry, Solanum species.

Occurrence and importance: This fungus can infect about 70 plant species. Of fruit trees, cherry and sour cherry are particularly at risk, but the disease also attacks apricot, causing disease of varying severity. The most significant damage occurs in cold and wet climates e.g. in Germany, the Czech Republic, Hungary, northern Italy and Austria. The shoots of young trees usually wither and die after a few years of dormancy (trees are susceptible for 8-10 years). Damage usually affects only part of the canopy; the entire tree rarely dies.

Symptoms and other disorders causing similar symptoms: The first symptom of Verticillium wilt is the shock caused by blocked water transport (water uptake and evaporation are interrupted). Leaves turn pale green and wilt; older leaves drop right away. The death of younger leaves is a slower process because as they remain on the tree for a long time while they wither. These symptoms are typically observed in June and caused by toxins produced by the pathogen, that may exert their effects at distance from the point of penetration.

Together with the external symptoms, a brownish-black discolouration of the vascular tissue can be observed when cutting 1-2 year-old branches.

Life cycle: Verticillium dahlea. is a soilborne pathogen that attacks plants through wounds and growing tips. Epidemics typically occur during dry spells in orchards with drip irrigation. The fungus may also be taken up by plant roots by means of osmosis. Overwintering structures (mycelia and other structures) penetrate woody parts and then enter the xylem, where the fungus blocks water and nutrient transport. The disease may kill some or all the trees in 1-2 year-old orchards. It is found on roots, the root collar. It is also found in nursery stocks. The pathogen prefers heavy, poorly ventilated, waterlogged soils, so orchards should not be planted in such areas. Rotating varieties and running a deep ripper in the area before planting may reduce pathogen populations. If the stone fruit orchard is planted after a susceptible herbaceous plant, the presence of the pathogen should be considered.

Control: Prevention is the key to controlling this pathogen. Always use certified propagating material. Diseased trees must be removed and destroyed. Remove susceptible dicots from young orchards. Prevent wounds on plants by keeping field mice, wild rabbits and deer out of the orchard. Apply chemical soil disinfection and control root and root collar pests. Biological preparations containing the antagonistic fungus Trichoderma and antagonistic bacterium Bacillus megaterium may be used for the preventive treatment of young trees. The product is absorbed by the plant, exerts a systemic effect, and does not decompose in the presence of light.

Host range: cherry, sour cherry, peach, apricot, plum.

Occurrence and importance:
This pathogen causes serious disease in gardens and plantations. The symptoms of the disease may occur on flowers, shoots, twigs, short producing sticks and fruits. The pathogen overwinters in fruit mummies interwoven with mycelium, but it is also possible that conydia survive the winter. Flowers can become infected even at a very early stages when the buds are white. If during the blossoming period the weather is cold and wet and there is considerable fog the conydia rapidly germinate on the stigma. The germ tube from the conydium then enters the stigma channel and destroys the whole flower. Infections in flowers rapidly spread into the shoots and destroy them in a couple of weeks. Chronic disease may develop on older branches. A similar process occurs in infections occurring through fruit wounds.

Source G Vétek, 2017

Source G Vétek, 2017.

Disease cycle: If the weather is cold and damp during bloom or if there is a lot of fog in the morning, the trees get infected through the flowers within hours. The fertilised conidial mycelium penetrates deep into the flower through the stylar canal and kills the entire flower. The mycelia spread rapidly to the shoots and reproductive parts. The mycelium developing from the conidia can also penetrate via the wounds caused by insect feeding, hail or sandstorms.

Source: https://agro.bayer.co.hu/termekek/karositok/korokozok/?id=113

Monilinia fructigena causes Monilinia disease in apples, pears etc., but in recent years has been causing severe problems in stone fruits. It infects the fruit through its thin skin when almost mature, or via cracks, wounds and insect bites. The entire surface of the infected fruit becomes covered with rotting brown spots and scattered pustula (exogenous stomata). Some of the infected fruit fall off, while the rest mummify and remain on the tree during the winter. Continuous grey mould mats appear at the point of penetration. The two species may be distinguished easily by their symptoms, as M. laxa forms concentric colonies of a milky coffee colour, as if made with a sewing machine. If the weather is wet during harvest, infected fruits rot and fall off, while in dry weather they remain on the tree and mummify. In some cases, the rotten fruit blackens; nevertheless, these will also develop the typical pustules, only later. The mycelia overwinter in mummified fruits and produce conidia in early spring, initiating infection through the flowers. This infection cannot be distinguished from that by M. laxa.

 

Control: It is essential to remove, collect and destroy mummified fruits and dead shoots. In cherry production, appropriate cultivation techniques should be applied (site selection, crown shape, nutrient management, resistant varieties, appropriate rootstock etc.). Chemical treatments should be scheduled at the stages of white bud, full bloom and petal fall can. Antagonistic fungi and bacteria against Monilia have shown promising results such as Trichoimmun, Trichoderma horsianum and Bacillus subtilis.

Stunted growth, mosaic mottling, distorted leaves and fruits (the latter with wrinkles and chlorotic or necrotic spots), reduced fruit size and the abortion of flowers have also been reported.

Transmission & Epidemiology: PVY is transmitted in a non-persistent manner, by at least 25 aphid species. Its most efficient vector is Myzus persicae (the green peach aphid), which colonises peppers. As Myzus persicae is able to retain the virus for more than 6 days.

Control:

• Remove/ destroy potato crops after the final harvest in order to minimise PYV spread to new crops.
• Destroy volunteer plants before planting to reduce PYV inoculum and aphid populations
• PYV-resistant varieties

 

Throughout insect vectors:

• protect hotbeds with nets to prevent early infection
• use coloured sticky traps to monitor vector populations
• control of the vector population by chemical, biological and/or integrated means
• oils

 

  

CUCUMBER MOSAIC VIRUS

Pathogen: Cucumber mosaic virus (CMV)
Host range: CMV has a very wide host range that includes more than 1000 plant species
Distribution: worldwide

Importance: CMV causes severe damage during flowering and in the yield. Its impact depends on the geographical area. Although it is distributed worldwide, the most significant losses are experienced under warm climates.

Symptoms: Young leaves often display a light-yellowish green mosaic mottling, which may develop into diffuse chlorotic lesions. Leaves get distorted, with curled main veins. On old leaves, chlorotic and/or necrotic concentric rings may appear, as well as irregular spots.

Fruit symptoms include reduced size, dark green spots, irregular ripening and/or sunken lesions with necrotic centres. The extent and severity of CMV symptoms is significantly affected by the age of the plant at the time of infection. When infected young, plants typically develop severe symptoms, including mosaic mottling, small and deformed leaves and reduced fruit size. In some cases, the disease may destroy the entire crop.

Transmission & Epidemiology: CMV is transmitted in a non-persistent manner by more than 86 aphid species. Myzus persicae (the green peach aphid) and Aphis gossypii (the cotton aphid) are two of the most efficient vectors.

CMV may also be transmitted by seeds.

 

Control:

Use virus-free propagative material (seeds, etc.)

Control of weed reservoir hosts

• eliminate weeds in the vicinity of the crop
• remove and destroy infected plants.

Chemical control of aphids

• protect hotbeds with nets to prevent early infection
• use coloured sticky traps to monitor the vector
• control of the vector population: chemical, biological and/or integrated control

Resistance breeding

The most effective methods of control are growing resistant varieties.

 

 

ALFALFA MOSAIC VIRUS

Pathogen: Alfalfa mosaic virus (AMV)
Host range: AMV infects a wide range of hosts, with about 600 species in 70 families being susceptible. Occasionally it may infect woody species.
Distribution:

Importance: Worldwide occurrence, commonly it does not cause economically significant damage, but in some cases, it may significantly reduce yields.

Symptoms: The bright yellow mosaic mottling on young leaves and their increasingly severe distortion are easily confused with the symptoms of other viruses.
On fully developed leaves, characteristic AMV symptoms may be observed including white blotches arranged in a mosaic pattern. In the summer, these symptoms are less important.
Fruits may be distorted and reduced in size and develop necrotic spots. These symptoms are particularly pronounced in the late cultivars.

Transmission & Epidemiology: AMV is transmitted in a non-persistent manner by aphids such as Myzus persicae Sulz. It may also be transmitted mechanically or by Cuscuta species in the field.

Control: 
Reduce viral inoculum levels:

• eliminate weeds in the vicinity of the crop
• remove and destroy infected plants.

 

Vector control to minimise transmission:

• protect hotbeds with nets to prevent early infection
• use of coloured sticky traps to monitor the vector
• control of the vector population: chemical, biological and/or integrated control

Produce virus-free seeds (e.g., seed treatment)

Isolation from other hosts (pepper, tobacco, peas etc.)

 

 

CAPSICUM CHLOROSIS VIRUS

Pathogen:   Capsicum Chlorosis virus (CaCV). It is a tospovirus (prototype TSWV), a member of the genus Orthotospovirus (family Tospoviridae) only identified on the island of Crete in EU (Greece) (EFSA Panel on Plant Health (PLH), et al. 2022).
Host range: CaCV infects plant species in the family Solanaceae (i.e. pepper, tomato) and ornamental plants.
Importance: Trade of plants (excluding seeds), ornamental plants, cut flowers, fruits and movement of soil and machinery contribute to further spread of the virus and/or viruliferous thrips to larger distances.
Symptoms:  CaCV can induce severe symptoms on its hosts, mainly on leaves and fruits. Main symptoms are:  frequently consisting of chlorosis, mottle. The young leaves show chlorosis, curling and deformation, whereas chlorosis, ringspot and line patterns develop on old leaves.

Transmission & Epidemiology: CaCV is transmitted in a persistent manner by thrips (similarly to TSWV).

Control:

Preventive measures should be applied to hinder thrips-transmission (similarly to TSWV).

 

TOMATO MOSAIC VIRUS

 

Pathogen:  Tomato mosaic virus (ToMV). It is an important member of the genus Tobamovirus.
Host range: The host range of ToMV is very wide (similarly to TMV).
Importance: ToMV has been reported from countries where tobacco, tomatoes or peppers are grown. ToMV is of greater economic importance than TMV and mostly damages greenhouse crops.
Symptom: Its most typical symptoms include mosaic mottling, deformed leaves, necrotic lesions and stunted growth.
The symptoms of ToMV are similar to those of TMV, but more intense, including dark greyish-brown stripes on the stems and branches, followed by the necrosis of the foliage and defoliation.
Transmission & Epidemiology: ToMV (similarly to TMV) is transmitted mechanically, by contact between healthy and infected plants or contaminated objects, similar to the transmission of TMV.
It is also transmitted by seeds in the case of certain members of the family Solanaceae.
It stays infective in plant debris and on contaminated objects for a very long time (persistent).
It may also be transmitted by Cuscuta species. 

Control: 
Preventive measures should be applied to hinder transmission.

• use healthy seeds
• disinfect tools
• isolate crops, remove and destroy infected plants
• use ToMV-resistant varieties

 

 

CANDIDATUS PHYTOPLASMA SOLANI (PEPPER STOLBUR)

 

Pathogen: Candidatus Phytoplasma solani (Stolbur phytoplasma)
Host range: Stolbur is able to infect practically any solanaceous plant and has also been found in sugar beet and field bindweed.
Distribution:

Importance: Stolbur regularly damage crops in droughty years in southern, eastern and central Europe. There is no chemical method available to control it.

Symptoms: The most important symptoms are the reddish discolouration of green parts, upward rolling of leaves, yellowing and defoliation. Flowers are malformed, have green petals and may develop into leaves. The disease urges the development of numerous shoots while stunting growth. The plant does not produce fruit or fruits are deformed.

Transmission & Epidemiology: Stolbur overwinters on perennial weeds, especially in the rhizomes of the field bindweed (Convolvulus arvensis). Its vector is the cicada Hyalesthes obsoletus (sometimes other cicada species, too). The insects pick up the disease when sucking on infected plants.

Stolbur is transmitted in a persistent manner. Infected cicadas, flying from plant-to-plant actively spread the disease during the summer. The incubation period of stolbur is long, so usually it is first detected in pepper fields in the late summer. Its vector is polyphagous and thus infects further bindweed specimens, promoting the persistence of the pathogen.

Control:

Proper weed control is of paramount importance, not only on the field itself, but also in its surroundings. Take special care to eliminate field bindweed.

There is no stolbur resistant cultivar. Dealers and distributors may say otherwise, but this is misleading.

Monitor the insects in the crop. When cicadas are spotted (they are difficult to find unless they jump or fly right in front of your eyes), precautions should be taken.