Phytophthora is causing a variety of diseases on a very broad range of host plants. Most species of Phytophthora cause rot of roots and lower stems. Infected plants show at first some symptoms of drought, since the roots can not keep up with the evaporation. Other pathogens or other causes are able to enter the plant, which is finally causing the death of the plant. This pathogen is even able to destroy fully grown trees. The disease is known all over the world and is a serious problem in all kinds of cultivated crops.

Symptoms

On all hosts affected by Phytophthora root rot, many off the small roots are dead and necrotic brown lesions are present on the older and larger roots. On plants with soft roots and tubers like Cycads, the whole root system may decay, followed by a more or less rapid death of the plants (figure 1). The plants are most of the times attacked by the fungus at the border between soil and air. It causes a watersoaked and dark bark on the soilline. The affected area enlarges and it may encircle the whole stem, after which the lower leaves drop and later on the even the younger leaves drop.
The stem may even break and the upper portion of the plant dies.

Spread of the disease

Phytophthora sp. belongs to the group of plant pathogenic fungi named Oomycetes. This fungus is able to form zoosporangia. These are sporeforming structures. Zooporangia are limoniform and have rounded base and a prominent, hyaline papilla on the apex (figure 2). Zoosporangia become detached when ripe and drift off to germinate almost entirely by releasing zoospores. It is also possible for the zoosporangium to germinate directly by forming a germtube which enters the plant and forms a haustorium. In de soil and under moist conditions zoospores are formed.
Zoospores have a flagella which makes it possible for the spores to move. Temperature en humidity are very important for the spread of the disease. The fungus is also able to reproduce in a sexual way. The fungus is than forming

oospores. These spores are round and very thick-walled. The are very persistent and
are able to remain in the soil under unfavorable conditions for a long time. When these oopores germinate they form a sporangium also.

Control and prevention

Preventive:

Phytophthora sp. is favored by a high air temperature, high humidity, poor soil aeration, excessive irrigation and poor plant spacing.

- Start your cultivation with clean plant material or seed.
- Steam the soil or make sure your soil is clean.
- Promote the growth in your crop (fertilization, light,
temperature and humidity).
- Work hygienic.
- Use clean irrigation water.
- Keep your crop not too wet (Cycads do not like a very humid
soil anyway), promote soil aeration and use a light soil.

Curative:

There are several fungicides that work very well against this disease.

- Paraat (dimethomorf)
- Aliette (fosethyl-aluminium)
- Aaterra (etridazole)

Always follow the instructions on the label of the package of the fungicide.

Additional questions can be addressed to:

Relab den Haan B.V.
P.O. BOX 38
2290 AA WATERINGEN

Telephone: +31-152124143
Telefax: +31-152147594
e-mail: info@denhaan.nl