Description
ORIGIN AND GEOGRAPHICAL DISTRIBUTION. Both pests, Geosmithia morbida and its vector P. juglandis, are native to the North American continent (particularly the Southwestern United States and Northern Mexico) and were first reported in Italy in 2013 in an American walnut(Juglans nigra L.) plant in Veneto. Subsequently, the disease was also reported in other Italian regions including Piedmont, Lombardy, Friuli-Venezia Giulia, Tuscany and Emilia-Romagna.
In the United States, the pathogen is seriously threatening the survival and productivity of walnut plantations, with serious damage to local and national economies. Should the disease spread extensively in Europe and Italy, the damage could be just as great, given that the walnut historically represents a species with multiple purposes as an ornamental, timber and fruit species.
HOST PLANTS The disease can affect all species of the genus Juglans and Pterocarya. The greatest damage is found on the species J. nigra, while on the other species of the aforementioned genera the degree of susceptibility is variable. In Veneto it was also found on J. regia, whose susceptibility is currently being assessed.
BIOLOGICAL CYCLE OF THE VECTOR. The adults of P. juglandis are 1.5-1.9 mm long and are yellowish brown in colour. They reproduce in subcortical tissues and dig short galleries in the phloem in which they lay their eggs. The larvae are legless (apodous), C-shaped (melolontoids), with a white body and reddish-brown head. They develop in the outermost layer of wood where they dig tunnels. In spring, the adults of the insect flicker from infected plants to healthy plants where they begin to dig subcortical breeding galleries, carrying the fungal inoculum (consisting of spores or mycelium fragments) with them and thus contributing to its dispersion. In Italy, P. juglandis performs two generations per year and the adults are active between the months of May and October. Mature larvae, pupae or young adults overwinter under the bark of host plants; adults are particularly resistant to low temperatures (even below -15°C).
SYMPTOMS. Symptoms are visible all year round, but are most easily detected from the vegetative restart. Amongst those that can be observed at a distance there is the manifestation of desiccation of foliage and branches starting from the distal, higher and peripheral areas of the foliage, where the presence of dry leaves can be observed that remain attached to the branches with a so-called 'flag' bearing.
On the branches, on the other hand, tiny holes of around 1 mm in diameter (sometimes with sap outflow) can be observed close to each other, representing the entry and exit holes of the adult insects.
Lifting the bark reveals not only the insect's feeding and reproductive galleries, but also the necrotic areas caused by the fungal infection.
The fungus mainly develops in the cortical tissues where it gives rise to the formation of superficial cankers and rarely affects the cambium area. The holes and associated cancers can be very numerous (hence 'disease of a thousand cancers'). The cancers may be very extensive and may completely surround the affected organ, causing it to dry out. In a more advanced stage of the disease, the cancers may also develop on the stem, leading to the plant's death within three to four years after the first symptoms appear.
PATHWAYS OF SPREAD. Geosmithia morbida spreads through the vector action of P. juglandis . The insect can move by flight, covering distances of approximately 1.6-3.2 km per year, but can also be transported passively by wind, by means of locomotion or more easily by the movement of infected wood or scrap wood, as well as by the marketing of nursery material, thus covering long distances.
Quarantine regulations currently exist in the United States to reduce the risk of spreading the disease, but there are no regulations imposing restrictions on the trade of walnut wood or nursery material from the United States to EU countries. Even within the EU, no measures have yet been enacted at the supranational level, but only at the local level on the initiative of regions where the disease has already occurred.
CONTROL AND CONTENTION MEASURES. There are currently no known direct (preventive or curative) control methods against copper dieback in walnut. The Provincial Phytosanitary Service has activated widespread monitoring throughout the provincial territory, also through sampling of symptomatic wood in different sites with host plants.
SIGNS. Already reported in Europe and Italy, prevention and surveillance activities are carried out for both. For this reason, the Provincial Phytosanitary Service and the E. Mach carry out annual inspections in various contexts, such as nurseries, urban greenery, natural and agricultural areas, and sites at risk. The investigations are based on visual inspections of the plants and, if necessary, on sampling and laboratory analyses. To effectively protect the territory, however, the cooperation of every professional, but also of every individual citizen, is indispensable.
Since they are considered a serious danger both for urban greenery and for natural and fruit-growing areas, suspected cases must be reported immediately to the Provincial Phytosanitary Service (tel. 0461-495660; e-mail: serv.agricoltura@provincia.tn.it;pec serv.agricoltura@pec.provincia.tn.it)
REFERENCE LEGISLATION. Regulation (EU) 2016/ 2031; Implementing Regulation (EU) 2019/2072; Legislative Decree no. 19/2021. There is currently no specific legislation concerning soft Geosmithia.