Estudio de la telaraña del champiñón causada por Cladobotryum Mycophilum en cultivos españoles

Resumen

Cladobotryum spp. is a mushroom parasite fungus. Its occurrence in white button mushroom (Agaricus bisporus (Lange) Imbach) commercial crops generates the pathology known as cobweb. This name is associated to the white fluffy mycelium that grows over the casing and the infected mushrooms; in the first stage it resembles a spider web that quickly evolves towards a dense mass of sporulation. Another disease symptom is the cap spotting on the mushroom which provokes loss of quality and a significant drop in profitability during the crop cycle. Recent years have seen the widespread occurrence of cobweb disease in Spanish button mushroom crops. Indeed, the presence of the disease has increased notably, resulting in serious economic losses. To provide a global perspective on the impact of the disease, A. bisporus mushroom crops were surveyed over a two year period to estimate the incidence and severity of cobweb along the crop cycle and the season, analyzing the casing material used. Cobweb disease usually appears in Spanish button mushroom crops when they age. Crops in the third flush are much more likely to be infected than those of the second and first. An early occurrence can lead to widespread infection by the end of the crop cycle. The pathology may appear at any time during the year although its presence and severity is greater in autumn and winter than in spring and summer. No significant differences were observed concerning disease emergence through the casing materials analyzed during the field sampling. Twenty-five Cladobotryum spp. colonies were isolated from commercial crops showing disease symptoms, plated on PDA and preserved. Their evolution and growth in vitro, taxonomic characters and genetic material were studied. The colonies secrete a pigment that becomes deep red and lack the characteristic camphor odour associated to C. mycophilum. Their conidia are big, bacillus in shape and mainly uniseptated while the phyalides are tapered to the tip which is simple and regular. At room temperature, spores germinated two hours after plating on PDA, and the germinative tubes showed a high growth rate. BLAST analysis of the sequenced amplicons revealed the closest degree of similarity to four Cladobotryum mycophilum sequences deposited in the GenBank database. Phylogenetic analysis showed the highest evolutive proximity to several sequences of Cladobotryum mycophilum. Therefore, the causal agent of cobweb disease in Spanish button mushroom crops is Cladobotryum mycophilum (Oudem.) W. Gams&Hoozem. The pathogenicity of Cladobotryum mycophilum was evaluated by applying a conidial solution over several casing types of an Agaricus bisporus germinated crop. Three different peat cased blocks and one mineral cased were checked. The pathology appeared in the inoculated blocks, while control blocks remained healthy. The cultural practices carried out during the crop cycle were those suited to a peat-lime casing, and so it can be affirmed that mineral casing blocks suffered yield reductions that were not directly attributable to the disease. Of note is the fact that all the casing materials used showed infection. Disease-inoculated blocks suffered losses of between 1% and 13% due to cobweb. The sooner the pathology appeared, the greater the yield losses registered. The cropping surface colonized by the disease increased as the cycle proceeded, and the eralier the infection appeared, the larger the crop area colonized. Once an outbreak of cobweb is located within the facilities, the individual outbreaks must be controlled. Indeed, it is necessary to treat them prior to sporulation. When not properly treated, conidia spread within crops and will disseminate the infection resulting in higher losses. A thick damp paper splaced over the isolated patches is recommended to prevent the spread of the disease. The response of the colonies to selected fungicides (chlorothalonil, thiophanate-methyl, prochloraz-Mn and thiabendazole) was analyzed in vitro and in vivo. Two of the analyzed strains showed resistant symptoms in vitro towards the benzimidazole fungicides employed. A couple of isolates also exhibited some tolerance to prochloraz-Mn, while all of them were sensitive to chlorothalonil. Prochloraz-Mn was the active substance with the highest selectivity between pathogen and host. On the other hand, chlorothalonil showed the worst selectivity among the evaluated substances. Cobweb control by fungicides was also studied in crop trials cased with peat based material, treated with the selected fungicides and subsequently inoculated with Cladobotryum mycophilum. Cobweb was responsible for yield reductions of up to 17%. Very poor control was achieved by using thiabendazole. Thiophanate-methyl also proved to be ineffective in coping with cobweb disease, as did chlorothalonil. Prochloraz-Mn was, on the basis of the results, the most effective fungicide among those evaluated for cobweb control. It showed the lowest ED50, as well as the highest selectivity towards the pathogen. Prochloraz-Mn was likewise the best treatment to prevent the colonization of casing surface by the disease, providing an accurate BE of the crop. In conclusion, mushroom cobweb caused by Cladobotryum mycophilum is a common pathology within Spanish mushroom crops. Isolates of C. mycophilum Type II, resistant to the bencimidazole fungicides, have been detected. Cobweb provokes yield losses in the infected crops. Its presence and severity are more pronounced in autumn and winter, with a higher incidence as the crop cycle progresses. Although prochloraz-Mn is the most efficient fungicide to cope with this pathology, evidences concerning a potential threat to the appearance of resistant strains to this chemical agent are presented in this thesis.