Propagation Problems
Posted by Phytopath on Apr 1, 2011
Do your seedlings fall over and die? Or your newly taken cuttings turn brown and rot?
Losses can occur when propagating plants: many of these can be avoided if you are aware of the conditions that trigger or encourage disease.
The problems that can occur during plant propagation can be divided into two broad categories; soil-borne diseases and air-borne diseases and pests.
Therefore good hygiene should always be practiced in the area where you are propagating your plants.
Today I will cover the soil-borne diseases of fungi and nematodes.
Microscopic fungi can infect seed within 24 hours of sowing. The seed simply rots in the soil; this is known as pre-emergent damping off.
Rotting of the stem at ground level, commonly called damping off in seedlings or collar rot in established plants, is usually caused by a soil-borne fungus called Rhizoctonia. The plant sometimes falls over and at other times remains standing.
Root rot is another fungal disease where the roots start to rot from the tips and then travel upward toward the stem. One of the fungi responsible for root rot is called Pythium which can occur in both young and mature plants, the other is called Phytophthora.
Rhizoctonia
Rhizoctonia is most active in warm humid conditions, so ventilation is important.
The young roots of plants are particularly prone to attack and visual cues to look for include; a mass of mycelium threads that look like fine spider webs, on the surface of the pot and infected tissue often has a reddish-brown colour.
The fungus can survive on plant debris, weeds and in the soil, so keeping the area free of old plant material and weeds is important. 
Pythium
Pythium is a water mould and occurs when the rooting medium is over-wet. There are a number of different Pythium species which attack soft, young tissue. Softwood cuttings and newly germinated seedlings are particularly susceptible.
The fungus can penetrate the stem and enter the cells causing rotting of the tissues. It also has the ability to survive in soil without a host plant therefore is able to infect subsequent plants if the potting medium is re-used.
Phytophthora
Phytophthora prefers warm, wet conditions. These fungi grow rapidly when the temperature is between 20°C and 30°C.
Other favourable conditions include: poor aeration and drainage of the potting medium, excessive irrigation over a long period of time and cool, wet weather.
The spores are spread via irrigation water, tools and equipment and can overwinter for up to nine or ten years in the soil.
The root system and stem base of cuttings are prone to attack. Lesions can be seen at the base of the stem when the bark is removed.
Visible symptoms include foliar wilt, yellowing or dying back of foliage and eventually death.
Nematodes
Nematodes look like teeny weeny worms, not that you can see them with the naked eye.
They are spread by transferring soil already infested with adult nematodes and/or eggs, or in water or infected plant material.
They feed on plants causing swelling on roots (galls) and stems. Leaves can die and plants can show signs of stunting or poor development.
Different species of nematode can attack different parts of the plant.
As you can see from this short list, hygiene becomes extremely important when working with not only propagation material but general gardening practices.
