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Heat stress in the cultivation of Poinsettia
High greenhouse temperatures can heat up the substrate to the point of ‘cooking’ young roots making them susceptible to root pathogens such as Pythium. Excessive watering during periods of high heat leads to poor root growth. When heat-stressed, Poinsettia plants will also be more susceptible to attack by Rhizoctonia. At the soil line, this causes canker (stem rot) but it can also infect roots causing death or stunted growth.

Another symptom of heat stress is leaf distortion. This is most commonly seen after pinching as new branches and leaves begin to grow. The appearance of ruffled leaf tissue on the interveinal areas is usually the result of air flow when the leaf temperatures are very high. Excessive watering during periods of high heat will lead to weak stems that will later be more susceptible to breaking. If relative humidity is low, branching can be compromised by the hardeneing of the tissue. In the case of extended, extreme heat, the phytoplasma in the phloem can be lost, compromising the hormone responsible for free branching. Research suggests that when daytime temperatures are 37°C or higher and night temperatures no less than 21°C, phyoplasma was removed from 10-20% of the plants. This effects plants at all stages of growth. More information: Rebecca Siemonsma E-mail: