Use of Phosphonate Salts to Control Chestnut ‘Brown Rot’ by Gnomoniopsis castaneae in Fruit Orchards of Castanea sativa

Abstract

The fungus Gnomoniopsis castaneae is the causal agent of the “brown rot” of sweet chestnut fruits. These days, this pathogen represents one of the main limiting factors for the sustainability of fruit production worldwide. Although heat treatment post-harvest is efficient in completely inactivating the pathogen, the application of appropriate protocols to control “brown rot” in chestnut orchards is required to help in reducing the latent population of the fungus in fruit tissues, and the consequent development of “brown rot” symptoms in the field before the post-harvest handling process. The present study aims to evaluate and compare the efficiency of products at a minimum environmental impact in experimental trials conducted in chestnut orchards in Central Italy for two consecutive years in 2019 and 2020. Phosphonate-based salts and, specifically, Zn-phosphonate were efficient in reducing the impact of the disease and the pathogen inoculum in fruits with an efficacy comparable to the fungicide Tebuconazole. A unique treatment at the blooming time produced the best results for both Zn-phosphonate and Tebuconazole, also giving indirect evidence of female flowers as a main site of infection. Phosphonate salts, and at first Zn-phosphonate, are highly effective to protect chestnut fruits from the ‘brown rot’ fungus G. castaneae. Its use in orchard management may complement the post-harvest heat treatment during the processing of fruits. Although a still ongoing debate on phosphonate salts use and efficacy in agriculture, they can be considered an optimal fungicide in chestnut orchards because of the low environmental impact when used at the recommended doses, the high translocability and stability, and the multiple mechanisms of action.

Potassium Phosphonate Induces Resistance in Sweet Chestnut against Ink Disease Caused by Phytophthora Species

“…Since scientific evidence shows that phosphonates have no nutritional role in the plant to justify their use as fertilizer, recently, the European Directive EU 1009/2019 has restricted their use, with a ban on their use in both conventional and organic agriculture. Therefore, any new formulation based on phosphonate will be registered as a pesticide/fungicide on target crops/plants [ 60 ]. Currently in Italy K-phosphonate is registered on seven fruit bearing trees, tomato and grape.…”

“…In addition, treatments of K-phosphonate must also consider the Maximum Residue Limit (MRL) recently issued by EU Regulation 2021/1807, which increases the limit for chestnut fruits to 1500 mg/kg. Considering the efficacy of K-phosphonate against other important pathogens on sweet chestnut, such as Gnomoniopsis castanea [ 60 ], further research focused on residue values after treatments is needed. Previous studies reported lower values than those issued by the EU regulation 2021/1807 [ 59 , 60 ].…”

“…Considering the efficacy of K-phosphonate against other important pathogens on sweet chestnut, such as Gnomoniopsis castanea [ 60 ], further research focused on residue values after treatments is needed. Previous studies reported lower values than those issued by the EU regulation 2021/1807 [ 59 , 60 ].…”

“…For the latter, only the highest K-phosphonate concentration of 280 g/L significantly reduced the lesion progress, suggesting some tolerance in this species. Development of resistance to phosphonate after prolonged exposure has been observed in some Phytophthora species [ 60 ], including P. crassamura , a close relative of P. megasperma [ 72 ]. This suggests that caution should be taken when relying on phosphonate as the only means to control Phytophthora diseases [ 73 ].…”

Mycotoxins and Other Secondary Metabolites Are Produced by Gnomoniopsis smithogilvyi When Confronted with Biological and Chemical Control Agents

“…Chemical control in chestnuts is not a current practice, and no fungicides are regulated for Gs control, although some studies have suggested the potential use of pyraclostrobin, difenoconazole, and phosphonate salts [14,15]. However, chemical fungicides generally depend on very limited and directed modes of action and are associated with resistance acquisition by pathogens.…”