Three‐way fungal interactions affect the potential biological control of Himalayan balsam, Impatiens glandulifera

Abstract

Himalayan balsam (Impatiens glandulifera) is one of the most invasive weeds across Europe. The rust fungus, Puccinia komarovii var. glanduliferae has been introduced as a biological control agent, but success has been patchy. Here, we investigated whether mycorrhizal and endophytic fungi can affect rust efficacy and plant growth. Over three experiments, we found that AM fungi and the rust alone or together consistently reduced plant growth, but this depended on the identity of species in the AM inoculum. Meanwhile, AM fungi increased infection frequency of the endophyte Colletotrichum acutatum. Rust inoculation had no detrimental effects on mycorrhizal colonisation or C. acutatum infection, but the latter two fungi reduced rust sporulation. However, plant size was reduced when all three fungal types were present, suggesting that a combined fungal inoculum offers a promising approach for the control of this weed.

The Invasive Plant Impatiens glandulifera Manipulates Microbial Associates of Competing Native Species

“…In this study, mycorrhizal fungal inoculation decreased the biomass of balsam when it was grown alone, but tended to increase it when it was grown in both inter- and intraspecific competition. In previous work, the Symbio inoculum has proven to be consistently antagonistic to I. glandulifera [ 18 , 36 ]. Furthermore, there appears to be a negative relation between colonisation levels in the field and plant size when the plant is invasive in the UK, but not where it is native, in the Himalayas [ 17 ].…”

“…However, the fact that some endophytes were found only when AM fungi were added while others disappeared with AM inoculation does suggest that there are interactions between the fungal groups within the plant. It has recently been found that the addition of the Symbio inoculum to I. glandulifera can increase or decrease the abundance of certain endophytes [ 36 ]. Clearly, the fungal identity and experimental location are likely to be two critical factors determining the outcome of future experiments involving these fungi.…”

“…Thus, the effects of I. glandulifera may be indirect; alteration of soil nutrient levels or reduction in AM fungal colonisation of C. arvense may lead to lower endophyte infection levels, as AM fungal presence has been shown to increase leaf infection levels of some fungal species [ 18 ]. In particular, AM fungi can increase infection levels of C. acutatum in I. glandulifera [ 36 ]. It may be no coincidence that this fungus was only detected in U. dioica and C. arvense when these plants were growing well away from I. glandulifera.…”

“…If a similar effect occurs with other fungi, this could lead to higher endophyte infection in the more mycorrhizal plants growing far from I. glandulifera. Two recent studies involving I. glandulifera [ 18 , 36 ] appear to be the only current examples of AM fungal effects on unspecialised endophytes in forbs [ 56 ], though there are a few examples of AM fungi altering foliar endophyte performance in grasses. Notwithstanding the fact that clavicipitaceous endophytes in grasses are taxonomically and biologically different to the fungi in forbs [ 29 ], the presence of AM fungi can enhance or reduce levels of Epichloe coenophiala, dependent upon the identity of the mycorrhizal fungal species [ 57 ].…”

“…We have previously suggested that adding mycorrhizal inoculum, such as the Symbio product, to field soils might enable native plants to compete more effectively with I. glandulifera [ 18 , 36 ]. Although in the present study the inoculum failed to reduce balsam growth in the competition treatments, the results presented here do not necessarily contradict this suggestion.…”