Dead arm, sometimes grape canker, is a disease of grapes caused by a deep-seated wood rot of the arms or trunk of the grapevine. As the disease progresses over several years, one or more arms may die, hence the name "dead arm". Eventually the whole vine will die. In the 1970s, dead-arm was identified as really being two diseases, caused by two different fungi, Eutypa lata and Phomopsis viticola.
Hosts and symptoms
Dead arm is a disease that causes symptoms in the common grapevine species, vitis vinifera, in many regions of the world. This disease is mainly caused by the fungal pathogen, Phomopsis viticola, and is known to affect many cultivars of table grapes, such as Thompson Seedless, Red Globe, and Flame Seedless. Early in the growing season, the disease can delay the growth of the plant and cause leaves to turn yellow and curl. Small, brown spots on the shoots and leaf veins are very common first symptoms of this disease. Soil moisture and temperature can impact the severity of symptoms, leading to a systemic infection in warm, wet conditions. As the name of this disease suggests, it also causes one or more arms of the grapevine to die, often leading to death of the entire vine.
Disease cycle
Dead arm of grapevine is caused by an ascomycete fungal plant pathogen. This pathogen produces sexual spores in the teleomorph stage and asexual spores during the anamorph stage. When the pathogen is in the teleomorph stage it is referred to as Cryptosporella viticola and during the anamorph stage is it called Phomopsis viticola. The teleomorph stage of the disease cycle does not occur in nature and involves sexual combination of the antheridium with the ascogonium to produce ascospores, allowing for genetic variation. The ascospores are encased in an ascus, which is further protected in a survival structure called the perithecium. Ascospores can be dispersed over long distances in the wind, but can also be mechanically transmitted or disseminated in rain. The anamorph stage is known to occur in nature and produces the main inoculum associated with this plant pathogen. During favorable conditions, conidia are released from infected lesions on the leaves or fruit and dispersed to other plants through rainfall or wind. Pre-existing wounds on the plant from annual pruning or insects allow the pathogen to gain entry into the next plant. However, if wounds are not present, the conidia can germinate to produce an appressorium to directly penetrate the plant. Once new plants are infected, conidia are produced throughout the season as the secondary cycle of this polycyclic disease. Phomopsis viticola overwinters as pycnidia until favorable conditions arise again.
Environment
The severity of dead arm in grapevine varies greatly between growing seasons. Fungal pathogens depend on moist conditions, causing the intensity of disease outbreaks to increase in wet environments. As the amount of rainfall changes between the seasons, so does the amount of pathogen present in the field. Prolonged rainfall early in the season has been correlated with greater disease outbreak. Temperature has also been shown to influence the infection rate. It has been found that the pathogen experiences the fastest rate of reproduction between 23 °C and 25°. Although temperature is important, the amount of rainfall has a greater impact on this pathogen because rainfall is an effective method of conidial dispersal. The conidia of Phomopsis viticola can also be dispersed through sprinkler irrigation and agricultural runoff. It has not yet been determined if an insect vector for this pathogen exists.
Use in wines
Although the dead-arm disease is usually looked upon as a malignant disease that often cripples one or more vines, some wine estates have discovered that the arms that are still alive when dead-arm has struck yield a very flavorful wine. One such vineyard belonging to Australian wine producer d'Arenberg have marketed this "Dead Arm" Shiraz, which has received high wine ratings among various wine critics.
Eutypa dieback
Eutypa dieback is caused by Eutypa lata which infects fresh pruning wounds when there is adequate moisture on the vine, such as just after a rain. The fungus also attacks many other hosts such as cherry trees, most other Prunus species, as well as apples, pears and walnuts.
Phomopsis leaf
Phomopsis leaf, also called cane spot or fruit rot disease, is caused by Phomopsis viticola.
