NECTARINE AND PEACH DISEASE MODELS
MONILIA SPP. ON PEACH AND NECTARINES
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Symptoms of the brown rot disease are the blight of the blossom as well as the green tip of twigs due to the penetration of the pathogen into the open blossom through the stigma of pistils or anthers. This usually results in wilting of the whole part of a one-year-old twig. The leaves start to hang down, later they become brown and rigid, but usually do not fall down on the soil, they remain on the tree until the spring of the next year. Sometimes, especially under humid conditions droplets of gum are visible, which are symptoms of colonisation of the fungus as well as the established cankers.
The infected fruits are covered by putrefactive spots, from which warty sporodochia (hyphe) with conidia of the “summer” form appear. Additionally, in late autumn and winter, the fungus produces sporodochia of the “winter” form on infected twigs. With time, severely affected fruits become mummified. The mycelium growing in such mummies gradually aggregates into sclerotia. Such fruits remain on the tree during winter.
Temperature and wetness duration are important environmental factors, determining the infection incidence of M. laxa on blossoms. Monilia laxa is well adapted to the relatively low temperatures during spring and cause infections at temperatures as low as 5°C within a very short period of wetness duration. The infection of the active bloom trough the stima does not need very much leaf wetness. Leaf wetness is only needed for germination of the conidia. Therefore infection of the young fruit needs longer leaf wetness periods. To infect the young fruit an appressoria has to be formed and free moisture is needed to build up the pressure to form the infection peg to enter the epidermis cell. With maturity of the fruit small scars on the fruits allow an infection without infection peg again and the needed leaf wetness duration becomes shorter again.
In FieldClimate the Infection, as well as the severity of infection, is calculated by the factors of rain, leaf wetness, relative humidity, and temperature.
SHOT HOLE INFECTION ON PEACH AND NECTARINES
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Shot hole is caused by the fungal pathogen Wilsonmyces carpophilus (formerly known as Stigmina carpophila).
Most commonly affected are apricot, peach and nectarines, but the disease occurrs on all stone fruit.
The fungus is able to infect all stages of the plant; the leaves, the twigs , the blossom and fruits.
Infected leaves show small brown spots with reddish margins (about 1 mm diameter), these spots expand to larger circular lesions (about 3mm diameter). These spots dry and fall out of the leaf, giving a “shothole appearance”. The infected twigs show clear- cut brown margins with a necrotic center, which does not drop out, but ooze large amounts of gum. Further on the lignification of infected twigs is hindered and the lesions will grow into cankers. In severe cases, premature defoliation of the tree may result.
Fruits show firstly small circular, deep purple spots. As the disease progresses, the symptoms differ according to fruit type. On apricots the spots become brown, raised and rough, giving the fruit a scabby surface. On peaches and nectarines, the scabs develop into deep indentations. Infected fruits have spots of gum and in severe cases cracks in the skin.
The fungal pathogen overwinters in infected buds and cancers on twigs and branches.
It is able to infect leaves, stems, and fruits during cold, rainy weather periods in spring and autumn. Rain periods to infect healthy plant organs are needed.
The fungus is able to persist several years in the cankers or buds of infected twigs. Whenever conditions are favourable it may continue to grow, even during wintertime. In springtime the conidia are splashed by rain to flowers and young leaves and infect them. In unfavourable periods (dry conditions) the conidia are still viable for several months. Rain is necessary for dispersal and humid conditions are needed for germination. The fungus is able to grow above 2°C.
In FieldClimate we calculate the Infection of the Shot hole disease in dependence of rain periods, leaf wetness, air temperature and relative humidity. The risk of a light, moderate and severe infection is determined (if a light infection is 100% the requirements for an infection of the disease in the field have been fulfilled).
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Peach leaf curl (fungal pathogen: Taphrina deformans) is a fungus disease that can cause severe early defoliation and crop loss on nearly all peach and nectarine cultivars.
The most common and striking symptom of leaf curl occurs on the leaves (foliage). Infected leaves are severely deformed and often display a variety of colors (light green and yellow to shades of red and purple). The fungus causes the meristematic cells at leaf margins to proliferate quickly and randomly, which results in the leaves becoming variously wrinkled, puckered and curled. As these infected leaves mature, naked asci containing ascospores of the pathogen are produced on the surface giving them a dusty appearance, after which the leaves turn brown, shrivel, and drop from the tree.
Many infected fruits drop early and go unnoticed; those that remain may become crooked at the stem end like a small yellow squash, while others develop reddish to purple and have “wart-like” deformities on the surface.
The pathogen occurs commonly almost wherever peaches are grown. The fungal pathogen overwinters as conidia (blastospores, “hyphal like” spores) in protected sites in the bark and around the buds. Primary infections occur during the early spring. Starting when the buds swell until the first leaves appear from the buds. Infections on young peach leaves occur at temperatures of 10°C to 21°C. Few infections occurs below 7°C. Infections appear mainly when rain wash the overwintered spores into the buds and cold temperatures lengthen the development time of the leaves (they are exposed for a long time to the pathogen before they are fully expanded and are able to resist the penetration of the fungus). If temperatures after bud swelling are warm and leaves develop quickly, infections rarely become established, even when spring rains occur. The wetness from rain (or other factors) for over 12,5 hours is needed for leaf infection but only when the temperature is below 61°F (is 16°C) during the wet period. Maximum infection occurs when trees are wet for 2 days or more, a frequent occurrence west of the Cascades. Although infected, symptoms may not appear if temperatures remain above 69°F (21°C). Fruit are susceptible after petal fall until air temperature remains above 19°C. Rainfall of 0,5 inch and wetness of 24hours are needed for fruit infection.
FieldClimateshows the Risk of a Taphrina deformans infection by calculation of temperature.
PEACH SCAB CLADOSPORIUM CARPOPHILUM
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Peach Scab, induced by the plant pathogenic fungus Cladosporium carpophilum, occurs on peaches, nectarines, apricots and plums.
The disease affects twigs, leaves and fruits. The most serious damage results from fruit infections. Twig cankers begin as small, reddish lesions on current season’s growth. These cankers expand slowly and may not be visible until mid-summer. The small cankers have irregular margins but do not cause sunken areas on the bark. Fruit lesions start as small greenish spots. These generally don’t appear until the fruit is half grown even though infection occurred earlier in the season. Older lesions are approximately 1/4 inch in diameter and develop a dusty or velvety green appearance. The numerous lesions typically are clustered near the stem end of the fruit. Extensive spotting can result in fruit cracks, which serve as entrance points for several fruit rotting fungi.
The fungus overwinters in lesions on twigs. Conidia are produced in the spring after petal fall and are windblown or splashed by rain. The conditions which favor disease development are temperatures above 16°C for spore production, over 10°C (optimal 22°C to 27°C) for spore germination, and between 2°C and 35°C for disease development. Most infection occurs at the time, when the shuck split, although the fruit remains susceptible through harvest.
On the twigs, the mycelium hibernates in the form of dark-brown spherical cells. It is possible that fungal conidia are able to overwinter. In springtime, conidia are produced from resting spores or mycelium. Through rain and wind, they will be carried to the leaves and fruits. Germination follows shortly and the fungus penetrates into the plant tissue. The infection starts after the petals fall, but symptoms are often not seen for some weeks. Inoculations and infections continue until the fruit matures.
Infection of the fruit: Here spores and mycelium are attached closely to the surface between the hairs, forming a mat of short, plump cells. The fungus does not penetrate into the flesh of the peach, but the close contact of the fungus with the outer cells allows the absorption of nutrition from the fruit through the unbroken walls. Evidently there is some injury to the outer cells.
The fungus is also able to infect the twigs, developing lesions as previously described. In these diseased areas, the pathogen is able to overwinter till the next season.
Temperatures between 7 and 24°C with leaf wetness periods from 19hours to 9 hours at 17°C to 19°C and increases to 17hours at temperatures of about 24°C (array with temperatures and leaf wetness hours for modeling). if the infection started a relative humidity of more than 90% is enough to increase the infection. if rel. humidity is above 70% then stops the infection.
The FieldClimate Model calculates a risk model of Cladosporium carpohilum in dependence of leaf wetness duration and temperature
RUST ON PEACH
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Peach 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. These 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. A few days afterward (depending on the temperature) the cankers mature and produce rusty brown powdery masses of specialized spores (urediniospores). This urediniospores are spiny and sharply constricted at the base. At the end of the season 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 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 dark brown to black and they produce two-celled teliospores. These leaf lesions are angular shaped, small size and rusty brown.
Fruit lesions develop during the growing season after the symptoms of the leaves. Firstly brownish spots with green halos on mature, yellow fruits are seen. When fruit reddens, 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 separated 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 infects 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 the first infection in the yards.
The fungus probably overwinters as mycelium in infected fruitwood 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. The University of California. Agriculture and Natural Resources.
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 occur 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.