Symptoms
First appear on only one or a few branches in the first year. The following year, other branches may show symptoms while the original branch may not. Symptoms appear in acute or shock years, and then commonly disappear. In early spring, foliation and blooming of infected branches may be delayed, with leaves remaining small with depressed fine lines (etching) and partial-to-complete rings on the upper surface. Areas of symptomatic leaves may become necrotic and fall out; giving those leaves a tattered look. Green fruit may show arcs and rings. Occasionally, large areas of bark are killed and show gumming.

Spread of the virus
Movement of virus in the orchard occurs through transmission by pollen to seed and to pollinated plants. Once transmission occurs, symptoms appear in two weeks to two years. On older, well-established trees, the infection may remain latent, with no apparent symptoms. Symptom severity depends upon the age and general condition of the planting, and on temperature, with leaf symptoms developing best between 20-24 C, and shoot dieback being more severe at higher temperatures.

Prevention and Control
Making sure that existing orchards are tested on a regular basis and are best kept separated from other infected orchards. Keep in mind that bees can fly for long distances during pollination. Two methods of control are usually implemented i.e. Certification programs that supply disease free material and orchard management practices to prevent tree-to-tree spread. This entails prompt removal of diseased plants; avoid the use of commercial beehives that have been working the area and the use of virus-free wood for top working. Obtaining virus free certified plant material from a reputable plant improvement organization.

Peach rootstocks grown from uncertified seed from peach canneries are important carriers of these viruses

Symptoms
This virus causes stunting and leaf malformation in 'Italian Prune'. It is usually latent in peach and nectarine and may cause chlorotic spots and rings in sweet cherry. If either rootstock or scion is infected, bud "take" drops 40 to 50 percent below normal for healthy stock.

Spread of the virus
The virus is borne in pollen and seed. As high as 80 percent transmission in seed of Prunus species has been reported. When infected pollen pollinates a healthy tree, it may also infect that tree, although frequency of this type of transmission tends to be much lower than seed transmission.

Prevention and control
Making sure that existing orchards are tested on a regular basis and are best kept separated from other infected orchards. Keep in mind that bees can fly for long distances during pollination. Two methods of control are usually implemented i.e. Certification programs that supply disease free material and orchard management practices to prevent tree-to-tree spread. This entails prompt removal of diseased plants; avoid the use of commercial beehives that have been working the area and the use of virus-free wood for top working. Obtaining virus free certified plant material from a reputable plant improvement organization.

Peach rootstocks grown from uncertified seed from peach canneries are important carriers of these viruses.

Symptoms
Causes ringspots on many Rosaceae spp. Growth can be retarded and yield reduced in sensitive cultivars from slight to severe (50%).

Spread of the bacterium
ApMV is transmitted by mechanical inoculation and by grafting. ApMV is not transmitted by seed.

Prevention and control
Very little spread is observed in orchards with infected trees. Most of the spread of this virus was attributed to root grafts. No natural vectors are known. Rouging of infected trees and the use of certified disease-free planting stock are effective control measures.

Spread of the virus
The virus is usually spread through contaminated plant material, mechanical inoculation as well as by grafting. This virus is not transmitted by seed.

Prevention and control
ACLSV can be controlled by using virus-free rootstocks and scion material. Newly established plantings remain free of the virus where clean planting material has been used. Virus free material can be obtained through heat-treatment, selection of spontaneous virus free shoots, or meristem culture followed by indexing.

Symptoms
The disease attacks most parts of the tree. Cankers on trunks, limbs and branches exude gum during late spring and summer. Leaves on the terminal portions of cankered limbs and branches may wilt and die in summer or early autumn if girdled by a canker. Occasionally, large scaffold limbs are killed. Leaf and fruit infections occur sporadically, but they can be of economic significance in years with prolonged wet, cold weather during or shortly after bloom. Leaf spots are dark brown, circular to angular, and sometimes surrounded with yellow halos. The spots may coalesce to form large patches of dead tissue, especially at margins of leaves, or the centres of the necrotic spots may drop out, resulting in tattered leaves. Infected leaves may abscise during mid season. On fruit stems, lesions are elliptical and brown with water soaked margins.

Infected leaf and flower buds may fail to open in spring, resulting in a condition referred to as “dead bud.” Small cankers often develop at the bases of these buds. Other infected buds open in spring but collapse in early summer, leaving wilted leaves and dried-up fruit. If blossom infection occurs, whole blossom clusters collapse as infection spreads into the fruit-bearing spurs. Blossom blight and spur blast are most likely in years when leaf and fruit infections are common.

Spread of the bacterium
The bacteria can survive from one season to the next in bark tissue at canker margins, in apparently healthy buds and systemically in the vascular system. Bacteria multiply within these over wintering sites in the spring and are disseminated by rain to blossoms and to young leaves. Bacteria of both pathovars can live in an epiphytic phase on the surface of symptomless blossoms and leaves from bloom through leaf fall in autumn. After leaves abscise in autumn, the bacteria may enter the tree through fresh leaf scars.

Outbreaks of bacterial canker are often associated with prolonged periods of cold, frosty, wet weather late in the spring or with severe storms that injure the emerging blossom and leaf tissues. Freezing can predispose the tissue to infection, but infection depends on the presence of wet weather during the thawing process. Free water on leaf surfaces and high relative humidity are required for at least 24 hours before significant leaf infection can occur following violent storms. Symptoms appear about 5 days later at temperatures between 21 and 27 degrees C.

Bacterial canker is much more severe on cold-injured trees and on trees growing in sites with poor soil drainage. Therefore, practices that minimize wounding and freeze injury and maximize good cultural practices will reduce the incidence of bacterial canker. Pruning tools can transmit the bacteria so these should be disinfected between trees if bacterial canker is present.

Prevention and control
Cankers should be pruned several inches below obvious infection during the winter when temperatures are freezing or below. The choice of cultivar is important, for some stone fruit are more susceptible than others to bacterial canker. Sprays during the growing season have not been effective in disease control. Chemical control of bacterial canker is based on protective copper sprays. These sprays, however, protect from initial infection but cannot prevent the canker phase once infection has begun. Also, there is reported resistance to copper in orchards that have consistently used copper. Stone fruit crops are sensitive to copper and injury is common

Copper-containing compounds may be of limited value for the control of bacterial canker because strains of P. s. syringae resistant to copper are common in orchards with a history of copper usage. Also, copper injures most stone fruit crops. Even on the more tolerant crop species, it becomes more injurious as applications are repeated.

B. Bacterial leaf spot, Xanthomonas arboricola pv. pruni

Disease
This is a plant disease, which occurs in both peach and nectarine trees. This disease, which in severe cases may cause a peach or nectarine tree to lose all of its leaves. Bacterial leaf spot also causes discoloured, mottled and spotted fruit and can also be the catalyst for peach and nectarine trees to have a small crop or under grown fruits. It is one of the most devastating and destructive diseases of the stone fruit. This leaf spot disease is also a prime attacker of both plum and apricot trees.

Symptoms
Damage from bacterial leaf spot in peach and nectarine trees begins on the leaves. The peach and nectarine leaves develop brown spots, which are angular rather than circular. These spots can sometimes be black, but are most often a light to medium brown in colour. The centres of the spots, which appear on the fruit tree leaves, may fall out, having disintegrated. However, often these spots simply cause the entire peach or nectarine leaf to turn yellowish, and then droop eventually dropping off and causing defoliation of the tree. The peach and nectarine fruits, themselves, may also suffer from a similar angular spotting pattern of disease and infection. Once the stone fruits become infected, they will show brown and black spots, which eventually become cracked and pitted in appearance. Once the disease becomes severe, the spots will sink in as the fruits decays. If your fruit trees are infected with bacterial leaf spot, you may also note the appearance of lesions on the twigs of the tree. Both peach and nectarine trees can develop these lesions, which will become oozy and sunken on their twigs. At the worst stage of infection, the entire peach or nectarine tree will defoliate leaving only infected branches and fruits unless the fruit has already dropped.

Spread of the bacterium
The bacterium over winters in the lesions, which develop on the trees. When spring begins, the bacterium begins seeping from the sunken lesions and is thus disseminated. Rainfall, splashing water and any form of watering can spread the bacterial ooze and infect nearby susceptible fruit trees. This organism is very fond of moist, wet regions and when heavy rainfall occurs the spread can be quick and deadly to peach and nectarine trees. If the disease becomes so severe that the peach and nectarine trees defoliate before early summer, it is likely that the entire peach or nectarine crop will have growth problems, as well as produce crops that are both poor in quality and small.

Prevention and control
Bacterial leaf spot is, unfortunately, a tenacious disease in peach and nectarine trees and there is no adequate chemical control for this disease. However, in early spring when buds first open you can try a preventative method to suppress bacterial leaf spot by spraying with basic copper sulphate. Unfortunately, this only suppresses, and does not destroy the bacterial infection. Another method of temporary control is to prune out regions with infected lesions or to cut entire branches. This may stave off further infection of the tree, but it is likely once it rains, or water splashes any region where the bacterium still exists that it will continue to infect the tree.

Symptoms
Apple trees infected with apple mosaic virus develop pale to bright cream spots on spring leaves as they expand. Apple leaves tend to be mottled with white to creamy yellow flecks, spots, and patches. These spots may become necrotic after exposure to summer sun and heat. Most commercial cultivars are affected, but vary in severity of symptoms. Except in severe cases, infected trees can still produce a crop; yield reductions vary from 0 to 50 percent. In some cultivars, bud set is severely affected. Large numbers of leaves may drop from the tree early. Extreme variation occurs in symptom severity as a result of cultivar sensitivity and strain virulence.

Spread of the virus
Very little, if any, spread is usually observed within apple orchards with infected trees. Most spread has been attributed to grafts. No natural vectors are yet known to spread this virus and no evidence of seed transmission has been reported.

Prevention and control
Eliminating known infected trees and the use of certified disease free nursery material are effective control measures.

B. Common Latent Viruses of Apple

Disease
A number of latent viruses have been identified in apple. More common latent viruses include apple chlorotic leaf spot virus(ACLSV), apple stem pitting virus(ASPV), and apple stem grooving virus(ASGV). These viruses also cause diseases in other fruit crops for example: apple chlorotic leaf spot virus causes pear ring pattern mosaic and has been found in all pome and stone fruit species; apple stem pitting virus is the causal agent of pear vein yellows.

Symptoms
As the designation "latent" implies, these viruses are symptomless in most commercial cultivars, but may cause symptoms in certain cultivars, scion/rootstock combinations, and ornamental varieties. Symptoms of apple chlorotic leaf spot virus may include chlorotic leaf spots, leaf distortion, chlorotic rings and line patterns, reduced leaf size, and stunting. Apple stem grooving virus produces symptoms on 'Virginia Crab' such as chlorotic leaf spots, stem grooving and pitting, union necrosis, and swelling of the stem above the graft union. Like apple stem grooving, apple stem pitting virus symptoms are also associated with 'Virginia Crab', and include stem pitting of xylem that generally stops at the graft union. Apple stem pitting virus also causes fluted (grooved) fruit in 'Virginia Crab', and epinasty and decline in 'Spy 227'. Although causing no marked symptoms in commercial apple cultivars, latent viruses may have a detrimental effect on the growth and cropping of some cultivars.

Spread of the virus
Very little, if any, spread is usually observed within apple orchards with infected trees. Most spread has been attributed to grafts. No natural vectors are yet known to spread this virus and no evidence of seed transmission has been reported.

Prevention and control
Eliminating known infected trees and the use of certified disease free nursery material are effective control measures.

Symptoms
Leaf symptoms that resemble a fan are very conspicuous on Mission and French Colombard. Infected vines have shortened and more irregular internodes. Lateral sprout development, double nodes, and stem fasciations cause a bushy appearance. Many berries shatter, and others do not develop beyond shot size. The disease affects berry set in grapes and can result in yield reductions of up to 80%, making it a highly important disease from an economic perspective. (Walker and Yin, 1998).

Spread of the virus
This virus is transmitted from grape to grape by the longidorid nematode Xiphinema index. The virus is not transmitted through grape seeds and has no natural weed hosts

Prevention and Control
Control of nematode vectors through chemical treatments or soil fumigants. Planting of virus-free nursery material in nematode free soils. Planting disease free material obtained from a certification scheme within the industry. Diseased vines should be removed and destroyed.

The etiology of leafroll disease is not fully understood, but it is believed to be caused by a complex of viruses. At least nine distinct viruses, all members of the closterovirus group, have been associated with the disease. The most prominent of these is grapevine leafroll associated virus III, a long, flexuous virus that is restricted to the phloem tissues of the grapevine.

Economical losses, however, are incurred by substantial reduction in yield, while poor fruit quality, e.g. reduced sugar content, has also been linked to these symptoms. A yield loss of approximately 20% occurs each year for as long as the diseased vines are held in the vineyard. Since leafroll does not kill vines, producers may be reluctant to remove affected vines because of the perennial nature of the grape and the cost of re-establishing vineyards

Symptoms
Disease symptoms are fairly characteristic, especially in red cultivars, where a typical interveinal reddening and downward rolling of leaf lamina occur. The leaves roll characteristically; yield and quality of the grapes are reduced. White cultivars develop only slight yellowish discolorations of the leaves

Spread of the virus
This virus is usually transmitted by an insect vector known as the mealybug, Planococcus ficus. The mealybug is a serious pest on grapevines. It has spread rapidly in South Africa and has been found in most of the grape growing regions. Once established in a region, it is difficult to eradicate. The mealybug can feed on roots, trunks, canes, leaves and fruit. It is more tenacious than other, more common mealybugs because it can have up to six generations per year and can over winter under the bark and on the shallow

The mealybug does damage to grapevines by exuding honeydew, which makes the leaves and fruits clusters sticky and also attracts ants. Sooty mould grows on the honeydew, which can make the fruit unmarketable.

Prevention and control
The natural spread in commercial vineyards are slow. To reduce the spread of the infection, removal of infected plants, control of the vector and strict testing measurements are required. Planting disease free material obtained from a certification scheme within the industry.