Better Yields Through Science
Grape Diseases & Pests
|Disease||Symptoms, Life-cycle and pest management|
|Anthracnose||Pathogen: Elsinoe ampelina
All succulent parts of the plant, including fruit stems, leaves, petioles, tendrils, young shoots, and berries, can be attacked, but lesions on shoots and berries are most common and distinctive. Symptoms on young, succulent shoots first appear as numerous small, circular, and reddish spots. Spots then enlarge, become sunken, and produce lesions with gray centers and round or angular edges. Dark reddish-brown to violet-black margins eventually surround the lesions. Lesions may coalesce, causing a blighting or killing of the shoot.
A slightly raised area may form around the edge of the lesion. Infected areas may crack, causing shoots to become brittle. Anthracnose lesions on shoots may be confused with hail injury; however, unlike hail damage, the edges of the wounds caused by the anthracnose fungus are raised and black. In addition, hail damage generally appears on only one side of the shoot, whereas anthracnose is more generally distributed. Anthracnose on petioles appears similar to that on the shoots.
Leaf spots are often numerous and develop in a similar manner to those on shoots. Eventually, they become circular with gray centers and brown to black margins with round or angular edges. The necrotic center of the lesion often drops out, creating a shot-hole appearance. Young leaves are more susceptible to infection than older leaves. When veins are affected, especially on young leaves, the lesions prevent normal development, resulting in malformation or complete drying or burning of the leaf. Lesions may cover the entire leaf blade or appear mainly along the veins.
The fungus overwinters in the vineyards as sclerotia (fungal survival structures) on infected shoots. In the spring, sclerotia on infected shoots germinate to produce abundant spores (conidia) when they are wet for 24 hours or more and the temperature is above 36F. Conidia are spread by splashing rain to new growing tissues and are not carried by wind alone.
Another type of spore, called an ascospore, is produced within sexual fruiting bodies and may also form on infected canes and berries left on the ground or in the trellis from the previous year. The importance of ascospores in disease development is not clearly understood.
Conidia are by far the most important source of primary inoculum in the spring. In early spring, when free moisture from rain or dew is present, conidia germinate and infect succulent tissue. Conidia germinate and infect at temperatures ranging from 36F to 90F. The higher the temperature, the faster disease develops. Disease symptoms start to develop approximately 13 days after infection occurs at 36F and at four days after infection occurs at 90F. Heavy rainfall and warm temperatures are ideal for disease development and spread.
Fungicide recommendations for anthracnose control consist of a dormant application of lime sulfur in early spring just prior to bud swell, followed by applications of foliar fungicides during the growing season. At 10 gallons per acre, lime sulfur will burn out overwintering inoculum on infected canes, but at this rate it will burn grape tissue so it should not be applied after bud swell. Spraying the ground at this time will also help burn out other inoculum residing on remaining fallen leaves, twigs and mummies.
|Measles (ESCA)||Pathogen: Nine species of fungi in the genus Phaeoacremonium. The perfect stage has been found for three: Togninia minima, T. californica, and T. fraxinopennsylvanica
Affected leaves display small, chlorotic interveinal areas that enlarge and dry out. Foliage symptoms are frequently called “Esca.” In red varieties dark red margins surround the dead interveinal areas. Severely affected leaves may drop and canes may dieback from the tips. Symptoms may occur at any time during the growing season but are most prevalent during July and August. On berries, small, round, dark spots, each bordered by a brown-purple ring, may occur. These spots may appear at any time between fruit set and ripening. In severely affected vines the berries often crack and dry on the vine or are subject to spoilage.
Measles are caused by several species of Togninia, a fungus that produces perithecia on grapevines in old, rotted vascular tissue. Ascospores are released from fall and winter into spring with rainfall; temperatures do not seem to be limiting for spore release. Ascospores reinfect the vine through pruning wounds. Wounds remain susceptible up to 16 weeks after pruning with susceptibility declining over time. The pathogen enters the current season’s vascular tissue and it is believed that symptoms are expressed in the same year that new infections occur. Symptoms are caused by a toxin produced in the vascular tissue and include both leaf striping and fruit spotting. Other symptoms that appear in May are shoot tip dieback and tendril dieback.
Another species of fungus, Phaeomoniella chlamydospora, that causes the disease is closely related to the species of Togninia listed above and is also an endophyte in grapevine. This fungus overwinters as pycnidia in 3-5 year-old pruning wounds and releases pycnidiospores with rainfall from fall through spring. The pathogen also infects the vine through current year pruning wounds and produces symptoms. With both pathogens, there can be a 50% reduction in shoot growth.
Measles is more prevalent in areas with consistently high summer temperatures and in areas with heavy spring rainfall. Generally, plantings that are 10 years of age or older are affected, although measles has been seen on fruit and foliage on younger vines. In table grapes, mark areas of poor bud break in spring. Examine these areas at harvest for disease symptoms.
Control can be achieved with lime sulfur. However, it is important that the product get into the cracks and crevices of the vine because that is where the fungal fruiting bodies reside.
|Phomopsis Cane and Leaf Spot||Pathogen: Phomopsis viticola
Phomopsis cane and leaf spot is the name for the cane-and-leaf-spotting phase of what was once known as dead-arm. Eutypa dieback is the name for the canker-and-shoot-dieback phase of what was also once known as dead-arm. The name dead-arm has been dropped. Growers should remember that Phomopsis cane and leaf spot and Eutypa dieback are distinctly different diseases and their control recommendations vary greatly.
The fungus overwinters in lesions or spots on one- to three-year-old wood infected during previous seasons. It requires cool weather and rainfall for spore (conidia) release and infection. Conidia are released from pycnidia (fungal fruiting bodies) in early spring and are spread by rain to developing shoots and leaves.
Phomopsis cane and leaf spot appears as tiny dark spots with yellowish margins on leaf blades and veins. Spots first show 3 to 4 weeks following rain. Leaf death may occur if large numbers of spots build up. Basal leaves with heavy infection become distorted and usually never develop to full size. On shoots, small spots with black centers similar to those found on leaves occur usually on a basal portion of the shoot. After spots lengthen a few millimeters, the epidermal layers of the shoots usually crack at the point of infection. Heavy infection usually results in a scabby appearance of the basal portions of the shoot. On clusters, spots similar to those that occur on shoots occur on the flower cluster stems.
Lesions on leaves, shoots, and clusters become inactive during the summer heat but rain just before harvest can cause light brown spots on clean berries and spots quickly enlarge and become dark brown. Berries may shrivel and become mummified. Infected canes appear bleached during the dormant season. Severely affected canes or spurs exhibit an irregular dark brown to black discoloration intermixed with whitish bleached areas. The black specks visible in the bleached areas are pycnidia that develop during the dormant season.
This disease is most severe where spring rains are common after bud break; moisture is required for infection. Infections generally occur when shoots begin to grow. Spores are released in large quantities from the overwintering pycnidia on diseased canes and spurs. These are splashed by rain onto early developing shoots and infection occurs when free moisture remains on the unprotected green tissue for many hours.
Spur and cane lesions provide most of the inoculum for new infections. Reducing the source of the disease is important. In table grapes, mark areas in the vineyard exhibiting poor bud break in spring. Later examine these areas for disease symptoms. A treatment of liquid lime sulfur before late fall rains will reduce the viability of pycnidia as well as reduce overwintering powdery mildew spores. Shoot and leaf infection is most likely during the period from bud break until shoots are 6 to 8 inches in length. Lesions appear three to four weeks after infection.
In all areas, spring foliar treatments are advisable if rainfall is predicted after bud break. Apply before the first rain after bud break and before 0.5 inch shoot length and again when shoots are 5 to 6 inches in length. It must be reapplied after significant rainfall in order to protect shoots up to 18 inches in length.
|Powdery Mildew (both A and B isolate)||Pathogen: Erisiphe necator
The fungus may overwinter as a group of thin threads, called hyphae, inside dormant buds of the grapevine and/or as small black bodies, called chasmothecia, on the exfoliation bark of the vine. Buds on new shoots can be infected 4 to 6 weeks after shoots start growing but not after bud scales become suberized. These new infected buds remain quiescent until the next growing season. The fungus infects developing buds during the growing season.
Under optimal conditions of mild temperatures and high humidity, a single spore can germinate, infect the plant, produce a new colony and a new crop of spores in 3 days. Temperatures over 85°F inhibit germination of conidia (spores). Susceptibility of the fruit drops rapidly after 8% – 15% Brix. Leaves and canes can be infected up to and past harvest.
Appearing in the early stages as whitish or grayish patches on the leaves and ultimately covering both surfaces when severe. Colonies are more easily observed in full sunlight. Later in the season the mildew turns darker and becomes peppered with minute black dots (chasmothecis). On the fruit, the fungus at first may appear grayish and whitish, but later it has a brownish russetted appearance. Infected fruit cracks and drops from the cluster. Even the blossoms can sometimes be infected causing them to dry up or fail to set fruit.
Applications for adequate control or suppression of Powdery Mildew are needed on a continued schedule depending on severity of the infection and migration of the new growth. In southern regions of the United States, or lower elevations, or in warm climates, grape laterals (flag shoots) and terminals grow rapidly often 4 to 6 inches per week and continued coverage is needed for adequate suppression or control of Powdery Mildew. User of the product needs to make careful observations for the timing of application for proper control of Powdery Mildew. Powdery Mildew can attack all aboveground parts of the grape plant. Spray the entire portion of the above ground parts of the plant.