The ultimate aim of the project is to find and introduce a co-evolved insect or plant pathogen that exclusively attacks Himalayan Balsam, which can be released into the UK to control the plant whilst leaving indigenous species intact, so that the ecosystems can be restored.
Through our research, we found one particular natural enemy, a rust fungus, which was observed causing significant impacts on Himalayan balsam and we believe it can do the same here. It has undergone extensive safety testing in our quarantine laboratory here in the UK and we have established that it is safe.
The rust fungus
Like all rusts, the Himalayan balsam rust is an obligate biotrophic fungus, meaning it can only live and survive on living tissue. The rust is an autoecious (meaning it completes its life cycle on Himalayan balsam), macrocyclic (all five spore types are present in the lifecycle) rust fungus, which infects the stem and leaves of Himalayan balsam throughout the growing season.
This rust has a complicated life-cycle: the rust has five different spore types, all genetically the same and all affecting Himalayan balsam! The first visible spore stage in the spring are the yellow aeciospores borne in tiny cups, which erupt from the stem of seedlings, below the seed leaves. Infected plants are clearly identified as the infection causes the stem to elongate, warp and bend as the rust develops within the plant feeding on the living stem tissue. The aeciospores are spread by wind and rain, and infect the leaves of Himalayan balsam. The aeciospores enter the leaf through the stomata in a film of water, produced by dew or rain, and develop within the leaf feeding on the internal cells. No symptoms for this infection are visible until chlorotic spots are seen on the leaves after 7/8 days and then brown pustules containing the next spore types, urediniospores, erupt from the underside of the leaf, after about 14 days. These spores are the ‘cycling’ stage and cause significant damage to the plants by reducing the photosynthetic area of plants. Many generations of urediniospores are produced on the plants throughout the late spring and summer. These spores are wind borne and hence are also the dispersal phase of the life cycle, allowing the rust to move between populations of the weed. Spores are capable of moving many miles in air currents. Following the onset of the autumn, the leaves of Himalayan balsam begin to senesce, which induces the formation of nearly black teliospores instead of urediniospores. The teliospores are the resting, overwintering spore stage of the rust, with thick walls and dark pigment to help them resist adverse environmental conditions. They have evolved this way so that the rust can survive in the winter months when Himalayan balsam is not around. When Himalayan balsam dies back, teliospores embedded in the leaves, fall to the ground to become part of the leaf litter, where they overwinter until the following spring, along with the seeds of the Himalayan balsam.
With the onset of the spring and an increase in temperature, Himalayan balsam seedlings germinate and the increased temperature also induces the germination of the teliospores. The teliospores germinate to produce the fourth spore stage, basidiospores, which are projected to from the teliospore onto the just germinating seedling. The basidiospore infects the hypocotyl of seedling of Himalayan balsam and grows within the developing plant to produce the fifth spore type, spermagonia which erupt from the epidermis of the stem. The spermatia produced in the spermagonia cross with compatible spermagonia structures to induce the formation of the aeciospores and the whole cycle repeats.