Diplodia seriata (grapevine trunk disease)

D. seriata has been associated with diseases such as fruit rot, dieback and cankers on a wide range of economically and environmentally important plants. There are too many hosts to discuss them all, so only a couple of economically important hosts are provided below, although it is likely that the symptoms of cankers and die back will be similar across many of the reported hosts.

On apple, the fungus affects a variety of plant parts including leaves, fruit and branches. One of the most damaging is the fruit rot phase known as black rot which causes the fruit of apples and pears to rot before harvest and in storage. The disease can cause latent infections of these fruits which do not become apparent until after harvest. The first visible symptoms of latent fruit infection are small black lesions (2-4 mm diam.) which are slightly sunken with a corky texture. These black lesions do not enlarge further and only give rise to a rapidly progressing pale brown rot 2-3 weeks preceding harvest. The active stage of the fruit rot can be seen in the orchard and is characterised by rot that has concentric zones of lighter and darker brown colours, later the rotted areas turn black. Fruits affected by this kind of brown rot are rapidly colonised within 3-5 days. The fungus also causes a distinctive leaf spot, known as frogeye spot. Leaf lesions are initially small, purple specks that enlarge to form spots 3 to 6 mm in diameter, these spots have light brown-to-grey centres which are surrounded by one or more darker rings of tissue and a purple border. Dark pycnidia of the fungus may develop in the centre of older leaf spots. Stem symptoms of D. seriata begin as slightly sunken, reddish-brown patches within the bark. These areas enlarge and darken to form cankers with sunken centres and raised margins. Cankers may also develop as a superficial roughening or cracking of the bark, especially at the margins, where the cankers girdle the twigs or branches a blight and dieback is seen. D. seriata is regarded as an important pathogen of apple in the USA (Stevens, 1933; Brown and Britton, 1986; Brown-Rytlewski and McManus, 2000) but as a weak secondary pathogen on the same host in the UK and New Zealand (Laundon, 1973).

On grapevines D. seriata is known to cause the death of spring buds, leaf chlorosis, fruit rot and trunk dieback, with brown, hard necrosis of the wood that appears as wedge-shaped necrosis in cross sections of the affected plant parts (van Niekerk et al., 2006; Urbez-Torres, 2011). Other symptoms include internal streaking and pith necrosis of wood, failure of graft union in young vines and cane bleaching (Urbez-Torres, 2011). D. seriata is one of the most cited Botryosphaeriaceae species occurring on grapevines worldwide and is frequently associated with the ‘black dead arm’ disease of grapevine (Larignon et al., 2001; Urbez-Torres, 2011). Recently Urbez-Torres (2011) proposed the name ‘Botryosphaeria dieback’ to include the increasing number of Botryosphaeriaceous species besides D. seriata that have been associated with most of the symptoms and diseases above. Reports of the virulence of this pathogen on grapes varies with some artificial inoculation studies (Spagnolo et al., 2017; Pinto et al., 2018; Reis et al., 2019) suggesting that it is a weak pathogen to grapevine and possibly takes advantage of weak or stressed plants (Qiu et al., 2016). These differences may be due to variations in virulence between strains, or they may be a result of the incomplete knowledge of the taxonomy of the genus, which in turn hampers accurate species recognition and identification. It is also possible that in species with a broad host range, such as D. seriata, virulence of any given isolate may vary according to the host that is being attacked.

Cultural and chemical management options for control of Botryosphaeria diseases are similar in many cropping systems including apple, blueberry, grape, peach and pistachio. Benzimidazoles, quinone outside inhibitors (QoI), and sterol biosynthesis inhibitors (DMI) are extensively used to treat the external symptoms of Botryosphaeria blight in apple (Brown and Britton, 1986), grape (Bester et al., 2007) and pistachio cropping systems (Ma et al., 2001; Ma et al., 2002). These products are applied either prophylactically, or as treatments applied to pruning wounds, as these serve as important entry points for infection. In addition to the synthetic chemical pruning treatments there are also several commercially available biological and botanical wound treatments. The biological products have already been outlined in the section ‘Notes on natural enemies’ and mostly make use of Trichoderma fungal antagonists that are painted on to the wounds. In addition to these several botanical products have been tested for their ability to manage D. seriata infections in grapevines, these products include chitosan oligosaccharide, garlic extract and vanillin. In field experiments all three were able to significantly reduce infection in pruning wounds by D. seriata and P. chlamydospora, with the most effective treatment being a mix of all three (Cobos et al., 2015).

Cultural control mostly relies on sanitation by reducing inoculum sources such as cankers, blighted shoots, mummified fruit, and pruning. In Californian vineyards delayed pruning is recommended as the current timing coincides with the highest periods of spore dispersal by fungi in the Botryosphaeriaceae.