(IPM) - Important steps taken prior to- and early in the growing season can be valuable for disease prevention

Disease development in a container nursery

Greenhouses are artificial environments and as such are initially pest-free. They don't have the traditional background pest levels of bareroot nurseries nor do they have normal population controls that exist in natural environments. Biological pests are either  introduced into greenhouses during any given growing season or they can be carried over after having been introduced during a previous year.  Biotic diseases are traditionally described using a pathology "disease triangle".  Once a disease organism enters the greenhouse, i.e. on seed as a seed-borne disease, carried in as spores, referred to as airborne inoculum or carried over from a previous year on old styroblocks, succulent seedling root and shoot tissues fulfil the host component of the disease triangle while the environment component not only favours growth of the disease, it may or may not predispose the host to attack by subjecting the germinant or seedling to unnecessary stress.

IPM Strategies

1. Aim to prevent the entry of disease causing organisms:

·  Clean old debris out of the greenhouse

·  Wash benches

·  Sanitize styroblocks

·  Ensure a clean water supply

·  Don't mix crops in one greenhouse if possible

2. Know what you might be up against:

·         Search SPAR for seedlot specific fungal assay results

·         Test old styroblocks for the presence of pathogens and for the efficacy of sanitization methods

3. Once you know what you might be up against -  you can lessen their impact:

·         Use running water during imbibition

·         Encourage rapid germination through temperature control

·         Rogue dead or dying germinants and destroy them

Introduced diseases and conditions affecting roots

The fungi Pythium, Fusarium and Cylindrocarpon are common root rotting organisms that are introduced to the greenhouse. These can be carried from season to season as inoculum on old styroblocks making block sanitation essential. Pythium can also be encountered in the water supply. Fusarium can be seed-borne and Cylindrocarpon can arise from contaminated containers and benches. In general terms, the impact of these fungi are lessened through cultural controls that encourage good media aeration and drainage. These controls are more difficult to implement once root disease is encountered - than are procedures used to control foliage diseases. Thus, it is very important to take precautions such as styroblock sanitation prior to sowing to prevent the chance of disease. Additionally, because it is very difficult to amend later in the growing season, it is extremely important to ensure the growing media has adequate aeration with good drainage properties, thus reducing the chance of exposing seedling roots to waterlogged conditions later in the season.

Introduced diseases and conditions affecting shoots

Common diseases that can affect shoots of young germinants include Fusarium, Sirococcus and Botrytis.  Once again, these organisms are introduced into the greenhouse environment. Fusarium can come in on seeds as well as air or water splash. Sirococcus is most often introduced as a seedborne disease but it can blow in from adjacent infected crops or from the nursery perimeter. Botrytis is introduced as airborne inoculum.

Knowing the potential for Fusarium introduced on seed to cause problems in your greenhouse can be gained by checking SPAR for fungal assay results. If a seedlot has high levels of Fusarium this can lead to damping off as well as Fusarium top blight or hypocotyl rot. Avoid stressing the seedlings to heat or drought.

Sirococcus, introduced as a seed-borne disease can produce spores on young germinants which can then infect adjacent seedlings. If SPAR results indicate high levels of Sirococcus on a seedlot and germinants are dying, rogue and destroy them which will prevent  more spore production. If spruce or hemlock germinants appear to be dying have them tested for the presence of Sirococcus.

It is impossible to prevent the introduction of Botrytis inoculum to the greenhouse. This organism is too ubiquitous. The strategy for disease control here takes place at the host and environment corners of the disease triangle. Try to ensure seedlings don't become too etiolated and succulent by maintaining adequate light levels. This will help prevent providing a suitable host for the disease. Use ventilated styroblocks to maintain airflow around seedlings and water early in the day to ensure foliage dries as fast as possible which will discourage an environment that Botryiis likes.