Downy Mildew on grapevines

Downy mildew, which is caused by the organism Plasmopara viticola, ranks amongst the most devastating vineyard fungal diseases. It is particularly prevalent in areas that are warm and wet during the vegetative growth stages of grapevine development, and can destroy bunches and cause defoliation of grapevine leaves, resulting in yield loss.

Symptoms

The fungus only attacks the green parts of the plant and are primarily seen on the leaves, but can also be observed on the bunches and shoots. Typical symptoms are:

LEAVES: Initially lesions on the younger leaves appear round, yellow and oily. While on older, more mature leaves the lesions are angular, reddish-brown in colour and along the edge of the leaves. The typical white, downy fungal growth can be seen on the underside of leaves following warm, wet nights. During these conditions the lesions, which occur on the leaves, dry out and appear reddish-brown with a yellow edge. Leaf defoliation can occur under conditions of severe infection.

BUNCHES: Infected young vines initially appear brown and oily, but after exposure to warm, wet nights, the white downy fungal growth develops – berries remain susceptible to infection until 6-8 mm berry size (a pea size). Infected berries become brownish-purple in colour, shrivel and sometimes fall off. These symptoms are sometimes confused with sunburn.

SHOOTS: Young green shoots can also become infected. The shoot appears brown and oily, after which fungal growth can appear on the infected areas. The shoots will eventually become necrotic and die.

Life cycle Plasmopara viticola primarily overwinters in infect-ed grapevine residues (leaves, bunches or shoots) as oospores, where they can survive for three to five years.

PRIMARY INFECTION: Conditions that favour germination of oospores are a minimum of 10 mm of rain and temperatures of a least 10˚C for 24 hours. In addition, following the 24-hour period the susceptible plant parts must remain wet for at least two to three hours. Spores are transported to the susceptible green parts of the grapevine by rain or water droplet splash. For successful infection to take place the organism establishes itself on functioning stomata and consequently, leaves and shoots of 10 cm or longer are vulnerable. Under optimal weather conditions (20-25˚C), the “oily patches” will become visible within five days of infection on young plant tissue.

SECONDARY INFECTION: Spore formation occurs after a warm, wet or very humid night. Sporangiophores with sporangia grow out of the stomata on the underside of the “oily patches” or lesions. The sporangia are spread via rain and/or wind and, once again, require two to three hours of free water for germination and penetration. The disease can spread rapidly, under favourable conditions – repeated rain showers or heavy dew in humid areas.

Management

Successful management of downy mildew relies on the prevention/management of the primary infection. Since the secondary infection spreads very quickly, it is difficult to manage downy mildew during this phase. For this reason, it is important to begin management of downy mildew in vineyards, using fungicides, as soon as shoots reach 10 cm in length, until the berries have reached pea-size stage. Disease outbreaks during this period can result in total harvest loss.

Chemical control

PREVENTATIVE CONTROL: This involves the appli-cation of fungicides before the primary infection period (i.e. prior to 10 cm shoot length and prior to rain and/or wet conditions). Fungicides are there-fore applied before the fungus has caused infection and long before any symptoms are visible.

POST-INFECTION CONTROL: This involves the application of a curative (restorative) product directly after the infection period, but long before the “oily marks” can be seen. Curative products are only effective if they are applied within two days after infection has occurred. This strategy is, however risky as repeated rain showers, soil that is too wet and limited spraying equipment can mean that the vineyards cannot be sprayed in time.

Recommended application intervals during conditions that are favourable for infection, is seven to 10 days for contact fungicides, and 10 to 14 days for systemic fungicides. Should favourable conditions occur during the pre-flowering to the pea-size stage, and especially when the disease proliferates, fungicides must be applied every seven days, preferably using systemic products until weather conditions become unfavourable for the disease. Good coverage of susceptible plant parts is critical. Rain and overhead irrigation lower the residue levels on the grapevine and application must be repeated if heavy precipitation occurs after application. Poor coverage must be avoided since even systemic products do not protect all plant parts. This is particularly true for the susceptible flower clusters that are often hidden in the thick canopy, and are not easily reached by the fungicide. Importantly, producers must ensure that they have enough spraying equipment so that vineyards can be sprayed within seven days. The equipment must be correctly calibrated to ensure that the required volume of product is applied to the target area. In this regard, tractor speed is also very important. Vineyard conditions that are too hot or too cold can also lead to poorly applied product. Specific vineyard practices (e.g. canopy management, weed control, irrigation and fertilisation) can help reduce and manage the intensity of the infection. The practices must be applied in such a way that overly vigorous growth is avoided, air flow is improved and humidity in the vineyard is kept as low as possible.

The downy mildew fungus can only cause infection when it’s in free water. Implementing these practices will assist in reducing the number of successful infections. This article was prepared from the material developed for the SATI Video Series Basic principles of table grape production, which is available electronically in English, Afrikaans, Xhosa and Sotho on the SATI website (www.satgi.co.za) or from the SATI offices.

The influence of environmental conditions on the occurrence of downy mildew can be summarised as follows:

• For primary infection the 10:10:24 principle is relevant. This means that at least 10 mm of rain, combined with a temperature of at least 10˚C for a duration of 24 hours is required for spores to be released, immediately followed by two to three hours of the grapevine canopy remaining wet.

• For secondary infection warm, wet nights are required. Spores germinate on the underside of the leaves after at least four hours of darkness, during which the temperature is higher than 13˚C and the relative humidity is higher than 98%. In addition, the grapevine canopy remains wet for at least two to three hours, before sunrise.

The following important principles with respect to fungicides must be considered: All products registered for control of downy mildew inhibit the fungus to an extent that ensures that it can be managed. Producers have a choice of product from two groups:

1. Broad-spectrum contact fungicides which are cheaper, but do not exhibit the same degree of effectivity, protection and rain resistance as systemic products; and

2. Systemic products that are more rain resistant and, consequently show more protection against infection. In addition, these products are more effective and some of them contain curative properties. Since fungi can build resistance to systemic products, it is important that they are applied as prescribed. In instances where rainy weather has been forecast, in particular during the critical pre-flowering and flowering periods, it is advisable that producers apply systemic products preventatively.