Utilizing the Mini-Horhizotron as a Tool for Assessing Plant Root Disease Severity in Floriculture Crops

Two experiments were conducted to assess the use of the mini-Horhizotron in measuring disease severity of Pythium aphanidermatum on the roots of bedding snapdragons and poinsettias grown in three different horticultural substrates. Plugs of Antirrhinum majus 'Snapshot Red' (bedding snapdragon) and Euphorbia pulcherrima ‘Angelica White’ (poinsettia) were planted in mini-Horhizotrons containing either a commercial potting mix, a substrate containing 70% peat moss and 30% pine wood chips (PWC), or a substrate containing 80% peat moss and 20% PWC. The plants were allowed to grow until healthy, white roots could be seen through the clear sides of the mini-Horhizotron. Tracings of the root system were taken using clear transparencies at the time of inoculation with Pythium aphanidermatum, and one month later at the termination of the experiments. The tracings were photographed, calibrated and converted to high contrast black and white images in Adobe Photoshop and uploaded into Cornell University’s RootReader2D software for total root length measurements. Results showed that snapdragons grown in the 80:20 PWC substrate had a total root loss of 6%, as compared to 48%  for the 70:30 PWC substrate and 81% for the commercial mix. Pythium infection was more severe for poinsettias, with total root losses of 71%, 87% and 91% in the commercial, 80:20 PWC, and 70:30 PWC substrates, respectively. Visual observations that were noted weekly during the experiment for both snapdragon and poinsettia provided evidence of the timing and severity of root disease symptoms. The ability to observe the root system in situ allowed for mycelium growth/presence to be documented as it occurred after inoculation, which is difficult to do when plants are grown in traditional containers. Other observations included fungal gnat (Bradysia spp.) larva, water-soaked and necrotic roots, and loss of root hairs. The results of this study illustrate how the mini-Horhizotron, when used in addition to other disease assessment techniques, can help provide a non-destructive assessment of root disease severity over time. The ability to view the rhizosphere and the accuracy with which root length can be measured suggests that the mini-Horhizotron could have broad applications in plant pathology research.