Horticulture Management Notes – November
November 1, 2020
Growing media alternatives
A growing media is a substrate which provides a structure for plant growth and establishment. At present peat is the most commonly used substrate in the horticulture industry. It is a suitable growing media as it is consistent in it properties. The use of alternative substrates will need to be considered more readily as the drive towards sustainability and environmental awareness heightens. With peat harvesting becoming more limited and concerns of environmental impact, a scarcity can be predicted.
The most common peat alternative substrates are:
- Wood fibre – a processed by-product of the timber industry with a wool like material in texture. This works best up to 30% of the mix and can improve air filled porosity of the media.
- Coir – a by-product of the coconut industry. The most commonly used peat alternative currently available and can be used up to 100% in substrates. It is used frequently as a complete substrate or as a majority percentage of a substrate. It has poor nutrient holding abilities but has excellent water holding capabilities.
- Composted bark -a process of biodegrading another by-product of the timber industry resulting in a compost material which works up to 30% of a substrate. It can be a dense and bulky material so a smaller percentage is added to most substrate mixes. There is an issue ensuring that all properties of the substrate remain consistent.
When moving from a peat based growing system to a peat reduced or peat free system it is important to carefully manage nutrients.
Fungal or bacterial spot
Leaf spots are caused by fungi and bacteria. They can seriously affect the quality of plants and are the most common problem encountered by ornamental growers. Fungal leaf spots are commonly caused by fungi that belong to the genera Alternaria, Cercospora and Colletotrichum, with bacterial leaf spots linked to the pathogens Pseudomonas syringae and Xanthomonas campestris.
The range of organisms affecting the plant can be narrowed down as in many cases plants have ‘signature’ diseases, for example cyclamen are prone to Colletotrichum infections. The signs (physical evidence of the growth of the pathogens) and symptoms (impact of the disease on the plant’s growth and development) can further narrow down the pathogen responsible. Signs of a fungal pathogen include the white hyphae and fruiting bodies (rust pustules) that can appear on the plants. Unfortunately, even though a fungal pathogen is suspected, no visible signs are detected as the pathogen has not developed sufficiently. One option to enhance the chances of seeing signs of fungi is to use a moisture chamber. This can be a simple box with a layer of moist paper in the bottom. If a piece of infected plant tissue is placed in the box and the box sealed and kept in a warm place, mycelia growth or fruiting bodies can appear within two days. This approach works best with spots or blights in the early stages of disease development.
The shape, appearance and smell of leaf spots can also be used to help differentiate between fungal and bacterial pathogens. Fungal leaf spots tend to be circular, with a target like appearance and many having a yellow halo around the circumference. Fungal leaf spots also tend to create dry, papery dead tissue and do not have any smell. In contrast, bacterial leaf spots tend to be angular, with the limit of their growth dictated by the leaf veins and the spots tend to be water soaked, have a slimy texture and often produce a fishy or sour smell.
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