About Blue Mold
Blue mold of commercial tobacco is caused by the fungus Peronospora hyoscyamy f.sp. tabacina, syn. Peronospora tabacina and is still one of the most destructive diseases on tobacco crops in semi-arid and humid climate zones. It was reported in Europe the first time in 1959 and spread to a disastrous epidemic at the beginning of the 1960th
Tobacco blue mold is a downy mildew disease. Nicotiana species are the only known hosts. Because blue mold is requiring living hosts to grow, it is classified as an “obligate parasite”. Although blue mold is an Oomycete, it is not known, that its sporangiospores produce zoospores; i.e. infections occur generally via direct germination of these sporangiospores/conidial spores on the host plant. The role of Oospores, the sexual bodies in the live cycle of blue mold is not clearly understood and it seems that they have, if at all, only a minimal impact on the spread of the disease.
The natural live cycle of conidial spores, the predominant source for the spread of the disease, is several hours to several days. It is assumed, that the original inoculum source for blue mold infections in Europe originated from North Africa from infected wild Nicotiana species year by year. It is not clear, whether the pathogen is capable to overwinter in infected plant debris in Europe, but the probability seems to be very low, because blue mold requires living host materials and usually frost is damaging remaining plant materials in the fields.
Blue mold spores need wet leaves for germination and infection. Therefore, cloudy weather conditions with high humidity and moderate temperatures favour the development of the disease. First symptoms of the disease are visible 5 to 7 days after the infection and sporulation can be detected at the day of first symptoms but it occurs usually the following night.
Infections can occur in seedbeds as well as in the fields. In Northern Europe tobacco producing countries, seedbed infections are seldom, whereas from Southern countries it was reported more often. Infected plants in seedbeds are light green and on the underside of the leaves, a grey to blue mold can be detected. Field infections exhibit typical yellow spots on the upper side of the leaves (Fig. 1 and 2), usually first at the bottom leaves then developing to the upper leaves, too.With the time, the yellow spots turn to brown, necrotic areas (Fig. 3). The typical downy grey to blue mold can be observed at the underside of infected leaves (Fig.4). Severe attacks of blue mold or changing weather conditions can result in systemic infections of the stem and the whole plant, resulting in brown streaking lesions at the stem, stunting of the plant and a disorder of the inflorescence (Fig. 5).
The aim of the Blue Mold Information Service is to compile and disseminate information about blue mold outbreaks in the European Mediterranean zone. It shall assist growers, extension services and administrations to assess the risk of blue mold infections in there region. Additionally, blue mold samples are analyzed at the University of Hohenheim/Germany during the crop season on their response to Metalaxyl (sensitive/resistant), a common used fungicide to control blue mold. These investigations have shown, that due to frequent Metalaxyl applications, the Metalaxyl resistant blue mold strain is predominating, whereas with no Metalaxyl treatments, the sensitive.