Wheat blast, caused by Magnaporthe oryzae pathotype Triticum (MoT) is a global threat for wheat. The first report of the disease was in Brazil, but wheat blast has spread to other South American countries, and there are currently reports of its presence in Bangladesh and Africa, indicating that it could have a worldwide distribution. Initially restricted to South America, the disease spread to Asia in 2016 by the introduction of contaminated seeds, raising the question: how is MoT transmitted from seeds to seedlings?
This pathogen attacks the spikes and can cause yield losses of 5-51%, but can destroy the whole crop under the right environmental conditions. Total or partial bleaching of spikes is the most significant symptom of wheat blast and could lead to grain infertility and/or failure of grain filling. At its worst, the disease can partially or completely damage the spike, resulting in no grain production. The fungus can also attack the leaves: the symptoms appear as diamond-shaped water-soaked lesions that gradually turn necrotic with the progression of time.
This fungus, during the period that the wheat is not being cultivated, jumps to secondary hosts, which are the common weeds that grow near to cultivated fields. It is known that short-distance dispersal is through conidia, but over long distances it is propagated by seeds, which do not always present the symptoms described. Therefore, seeds that are apparently healthy can harbor the pathogen and spread disease far from its origin. This risks epidemics in areas where the conditions for its establishment and development are favorable.
We studied the relationship between seed infection and disease symptoms on seedlings and adult plants. To accomplish this objective, we inoculated spikes of wheat (to a variety named Apogee) with transgenic fungal isolates. Two isolates of Wheat Blast fungus were used (PY15W and PY34W) and transformed using Agrobacterium tumefaciens– to introduce a gene encoding a red fluorescent protein (DsRed). These transgenic isolates were identified in experiments using hygromycin (antibiotic) resistance for selection, or by observation of DsRed fluorescence. By tracking their presence, from seeds underground to seedlings, we demonstrated that wheat blast can spread from seed to seedling.
Our experiments revealed that this pathogen is able to colonize wheat seedlings, systemically (through the plant), from infected seeds underground. Many of the shrivelled and/or discoloured seeds grew into seedlings with high levels of Wheat Blast fungus. Some infected seeds which looked healthy also germinated into plants showing high levels of infection. This proves that seeds which appear healthy can spread disease. These findings highlight the urgent attention needed to minimize the risks of accidental spread of this dangerous fungal disease of wheat.
Sergio I. Martinez, Alex Wegner, Stefan Bohnert, Ulrich Schaffrath and Analía Perelló published this study in Plant Pathology:
Partial bleaching of spikes with mycelium caused by Magnaporthe oryzae triticum pathotype.
alt=”Two wheat ears against a black background. The wheat plants are a dark green colour, but one of the wheat ears is significantly bleached from the second. This variety has long awms. From the third row of spikelets to the top of the ear, all of the spikes are white or yellow and look completely dry.”
All images used with permission of the authors.