# Brown Rot Caused by _Gnomoniopsis smithogilvyi_ (syn.
## 1 Introduction  
The chestnut is a fruit of great interest in many regions of the world, with a total production of approximately 2.3 million tonnes, with China being the largest producer, with 1,743,354 tonnes [1]. However, this country produces the species _Castanea mollissima_ Blume, while European countries produce _Castanea sativa_ Mill. In the worldwide and European chestnut productions, Portugal ranks in the seventh and fourth positions, respectively, with an annual production of 42,180 tonnes in 2020, distributed through 51,700 ha [1]. The production of chestnuts in Portugal has a significant economic impact, especially in the region of Tras-os-Montes, where the largest area of cultivation of the chestnut tree is concentrated (30,000 ha) [2]. However, in the last decades, the production of chestnuts in this region, as well as in other regions of the world, has greatly declined. Specifically, in the Tras-os-Montes region (Portugal), losses of 70% in the production of some chestnut varieties have been reported in some areas for the year 2022 [3]. Both abiotic and, particularly, biotic stress factors, such as pests (ex. _Curculio_ spp. and _Cydia_ spp.) and diseases (ex. canker), have been the main causes of this decline [4]. _Curculio_ spp. and, in particular, _Curculio elephas_ (an important pest of the European chestnut _Castanea sativa_[5]) may cause significant economic damages, approximately 25-30% per year in Turkey, as mentioned by [PERSON] et al. [6]. Furthermore, on average, 56.3 and 27.8% of the infested fruits contain one or two immutures (eggs and larvae) [7], showing the frequent presence of this chestnut weevil in the groves. More recently, when analyzing the loss of fruit traits due to damage by this weevil in four populations of healthy sweet chestnut trees in Turkey, [PERSON] et al. [8] verified that the fruit weight loss percentages varied between 25% and 32%, demonstrating again the production and economic losses that this pest can cause to producers. _Cydia splendana_ Hubner (Lepidoptera: Tortricidae) is another pest that chestnut producers face because the larvae feed on the fruits, reducing their quality and economic value. Significant attacks in Spain [9] and Hungary have been reported. Nevertheless, management strategies to control chestnut tortrix in Italy [10] and Hungary [11; 12] have been developed. Concerning diseases, the chestnut blight fungus _Cryphonectria parasitica_ causes cankers, lesions caused by the growth of mycelia within the bark tissue of the host plant, which in some extreme situations may result in the death of all plant tissue distal to the canker [13]. This disease is frequently controlled using hypovirulent strains [14]. However, some challenges must be surpassed because it is known that the presence of other fungi in cankers may inhibit the hypovirulent form of the pathogen more than the virulent form, intensifying the canker development [13]. Moreover, ink disease is caused by the oomycete _Phyophthora cinnamomi_, which is a dangerous pathogen that causes root rot, threatening the production of chestnuts worldwide [15; 16]. In the Tras-os-Montes region, this situation becomes even worse due to the newly introduced pest _Dryocosmus kuripillus_ Yasumantsu (Hymenoptera: Cynipidae) and, more recently, the opportunistic fungal pathogen _Gtomionopsis smithlogilvyi_ (syn. _Gtomoniopsis castaneae_) [4; 17]. [PERSON] and [PERSON] [18] reported that the origin of the disease is not known. In Portugal, the presence of this fungus in chestnuts was reported for the first time in 2020 by [PERSON] [19] and then by [PERSON] and [PERSON] [20]. Despite being an emerging disease in chestnuts, the presence of this fungus has already been reported in several European countries, as well as in Asia and Australia, being mainly responsible for the brown rot of chestnut tissues. The disease caused by brown rot in chestnut tissues is still difficult to understand. The fungus presence is not visible outside the fruit, even after it has already rotted inside. Nevertheless, _G.