First detection of Trichothecium roseum causing leaf spots on tomato in Germany.

Abstract

Tomato (Solanum lycopersicum L.) is one of the most popular vegetables in Germany and the demand for organically grown tomatoes in particular is growing steadily. In seasonal low-cost organic tomato cultivation, the leaf mould disease caused by Fulvia fulva is an increasing problem (Meyer et al. 2021). In 2020-2021, during a survey for F. fulva infected tomato leaves in Germany, untypical, circular, and dark brown necrotic spots were observed. To identify the pathogen, symptomatic leaves were collected from three different geographic locations in the north and south of Germany. Specimens were observed under a stereomicroscope and immediately processed. After surface disinfection of symptomatic leaf material, the leaf tissues were placed on potato dextrose agar (PDA) and subsequently incubated at 25 °C in the dark allowing mycelium to grow. Single spore isolates from each location were prepared for morphological and molecular analyses, as well as for pathogenicity determination. Three single spore isolates (JKI-GFP-22-010 - 012) showed moderate to rapid growth rates that reached 4 cm in diameter after 7 days. Based solely on morphology (S1), they were identified as Trichothecium roseum (Pers.) Link (Link 1809) by having pink dense mycelium with circular and rough edge colonies. The conidiophores were simple or branched (105 - 254 × 2 - 4 μm). Conidia were 2-celled, smooth, thick-walled, hyaline, ellipsoid to pyriform with slanting and truncate basal mark, and were produced in clusters, (10 - 18 × 7 - 9.5 μm). To afford the genetic data of these fungi, fungal DNA was extracted using a DNAeasy® Kit Plant Mini Kit (QIAGEN®, Germany) and then subjected to PCR using the primer sets as follows, ITS1/ITS4 (White et al. 1990) and LROR/LR6 (Vilgalys et al. 1990). The consensus sequences generated in this study were deposited in GenBank under the accession numbers ON787818 - 20 and ON787821 - 23, for ITS and LSU, respectively, and compared via NCBI-BLAST. Initial BLAST searches of both ITS and LSU regions revealed that the isolates share 99 - 100% homology with T. roseum. The morphology and phylogeny (S2) supported assignment of all isolates to T. roseum. To fulfil Koch's postulate, the pathogenicity of selected isolates (JKI-GFP-22-010 and 012) were tested by spraying spore suspensions (105 conidia ml-1) on adaxial surface of 21-day-old tomato plants cv. 'Moneymaker' (n=10). Plants sprayed with sterile water served as controls. All tomato plants were grown under greenhouse conditions (average 25 °C, 98% relative humidity). After 21 days, symptoms were identical to those of the originally infected leaves associated with T. roseum. As the disease progressed, necrotic lesions reached approximately 35 mm in diameter. The fungus was re-isolated and identified as T. roseum. No symptoms developed in the control plants. Trichothecium roseum has been detected infecting fruit, particularly on tomatoes, e.g. in Argentina (Dal et al. 2008), Brazil (Inácio et al. 2011), Korea (Yun et al. 2013), and Pakistan (Hamid et al. 2014). To our knowledge, this is the first report of T. roseum infecting tomato in Germany. As the pathogen was found in several locations in the south and north, it is likely already well established in Germany. To date, it seems to play a minor role in pathogenicity of tomatoes but should be monitored, especially with regard to climate change as there are now several recent reports from the temperate region. The disease development and interaction between susceptible host and pathogen should be concerned.