Nectria Twig Blight (Coral Spot) of Apple

At a glance, nectria twig blight (coral spot) may look to be shoot dieback or blight caused by other more prevalent apple diseases, such as fire blight or abiotic damage resulting from a break. On close inspection, one will find a canker at the base of the shoot or site of infection, which by late summer/early autumn is often covered with pink or salmon-colored nodules (sporodochia). Nectria twig blight is a sporadic disease that typically affects apple trees in temperate climates and has often been reported in production regions around the Great Lakes and eastern North America. Although historical losses caused by this disease have not been widely documented, the impact upon the health and yield of affected trees is considered trivial, and its economic impacts at an orchard scale is negligible.

Causal Agent

The causal agent of nectria twig blight is Nectria cinnabarina, an opportunistic fungal saprobe that attacks weakened or injured shoot tissue. Other species in the Nectria genus vary in terms of pathogenicity and aggressiveness, but as whole, they are capable of colonizing the tissues of hundreds of species of woody perennials. In particular, N. cinnabarina can be found on apple, pears, crabapple, and even maple and currants. In this regard, surrounding woodlands can be a source of inoculum for the pathogen. 

Symptoms & Signs

On apples, nectria twig blight is characterized by a pale wilted blight resulting from a girdling amber-brown canker at the afflicted shoot’s base. These cankers often occur at the site of a pruning wound, winter injury, an injured fruit spur at harvest, or at the junction between the new shoot growth and that from the previous year. This canker develops to compromise the vascular tissues and prevents the shoot from getting sufficient water and nutrients causing it wilt, wither, and eventually die (Fig. 1A). Leaves wilt and turn brown, remain soft to the touch, but are fastened securely to the shoot despite their fragile appearance. In the autumn months, microbial succession by other wood decay saprobes may cause the leaves to blacken, shrivel, and fall away (Fig. 2). As the remaining nutrients and moisture in the infected shoot begin to decline, bright pink or coral-colored sporodochia form at the site of the infection, and much of the basal surface of the blighted branch may be covered. (Fig. 3). Although not often easily distinguished, blackened sections of the canker under the epidermis may be covered with a recalcitrant fungal stroma on which the sporodochia develop. In some cases, bright red or crimson perithecia form on the stroma and have a tiny ostiole from where ascospores will be released. 

Nectria twig blight symptoms may appear as an injury such as a broken shoot (say, from being clipped by a tractor). Early-stage symptoms are commonly mistaken for fire blight due to a similar appearance of blight shoot tissue (Fig. 1B). Understanding the distinction between these two diseases is important to prevent rushed decision-making and wasted resources. Compared to nectria twig blight, which begins at the base of plant and is a pale wilted blight resulting from restricted water and nutrition, fire blight infections occur at the growing shoot tip and cause a black spreading necrosis along the length of the shoot as the causal bacteria advances down the shoot and into the central leader. In fire blight infections, as tissues become saturated with bacterial cells and exopolysaccharides, tissue will rupture with an ooze droplet. In direct contrast, there is no ooze resulting from nectria twig blight infection, and there typically is no appreciable spread of the fungus from the point of infection into the central leader (Fig. 2).

Disease Cycle and Epidemiology

Nectria cinnabarina survives as sporodochia, mycelium in woody tissues, or as the stromata formed on cankered tissues during the late fall. The fungus undergoes sexual recombination during the late winter and perithecia may develop on tissues in the early spring. As perithecia mature, ascospores are exuded as a curly white gelatinous tendril that may be rain-splashed or dispersed by wind to new sites of infection on wounded or compromised host tissues. Spores germinate to form mycelium, which invades the compromised wood obstructing the host’s vascular system, causing shoot tissue distal to the point to infection to rapidly wilt and die. By late summer to early fall, sporodochia form and generate asexual conidia, which can be rain splash and wind dispersed throughout the orchard causing infections on summer pruning wound sites, or on injured fruit spurs or pedicel scars made during harvest (Fig. 4). Sporodochia will remain visible on infected host tissue through the winter and may contribute to overwintering inoculum in the following spring (Fig. 3).

Management

Due to the minor impact nectria twig blight has on overall tree health and productivity, and the fact that the fungus is protected within the host tissues, chemical management in the form foliar fungicide applications may not be a cost-effective means to manage this disease. Instead, management techniques to control this disease should largely focus on prevention and eradication.

Maintaining tree health and vigilant pruning are the best means to prevent and manage nectria twig blight. Prune infected shoots on cool, dry days when possible and remove prunings from the orchard and/or burn them to minimize disease spread as cuttings will continue to release viable spores. At harvest, take care to minimize scarring on fruit spurs and clusters to reduce infection points for conidia produced in late fall. During the winter months, prune out visibly infected branches covered in sporodochia and remove trees if the point of injury is on the central leader. Explicitly resistant apple cultivars to nectria twig blight do not exist, considering that the disease has not been observed on many cultivars. Those with observed susceptibility tend to have large cluster-bud bases. Susceptible cultivars include ‘Anna’, ‘Ben Davis’, ‘Empire’, ‘Fuji’, ‘Granny Smith’, ‘Idared’, ‘McIntosh’, ‘Northern Spy’, ‘Rome Beauty’, and ‘Twenty Ounce’.