called the Latent Period, the period between infection and expression ie the time from
when a spore lands on a leaf and the time a lesion is produced with further spores. The
Latent Period varies enormously; in wheat, Leaf Blotch (Septoria triticii) has about four
weeks latent period. If a fungicide is applied that has only preventative but no curative
(orkick-back) action, it will prevent further infection but not eradicate the pathogen
already present. So if wheat has been recently infected with Leaf Blotch, but the
symptoms have not yet appeared, a strobiluronfungicide would not be appropriate, and
atriazole would be the type to use.
Strobilurons are a recently developed fungicide group. They are powerful synthetic
mimics of naturally occurring chemicals produced by the subterranean hyphae of some
fungi to prevent the development of other “rival” fungal pathogens competing for their
substrate. The first strobilurons to be developed had no kick back activity, but the new
generation does have kick back.
Triazole fungicides are amongst the most commonly used in Europe, and are both
curative and protectant, with a systemic mode of action.
Morpholine fungicides are another group which are contact fungicides only, and
have no kick back.
Sometimes fungicides are applied to a crop as a routine, preventative measure even
before any symptoms are seen, when certain diseases are known to usually occur every
year. This is normally done with high value crops such as tomatoes and other
vegetables, and also with Irish potatoes to control Potato Blight.
A famous old fungicide is known as Bordeaux Mixture. This is one of the oldest
and most effective of the fungicides, giving good protection against Potato Blight and
other diseases. It can be bought, or even made at home, as follows: Recipe—Dissolve
225 g of copper sulphate in about 23 litres of water. Make a creamy mixture of 150g of
quicklime mixed with a little water. Pour the cream through a fine sieve onto the copper
sulphate solution. Test the solution with a clean knife—if the blade comes out coated
with copper, add more cream to make the copper dissolve totally. This mixture should
be used up within no more than two days.
B. Viruses
A virus is a submicroscopic living organism, a single nucleic acid surrounded by a
protein coat. They do not produce any kind of spore but are spread from plant to plant
(or animal to animal) within their cells. They are cell parasites and are usually specific
to both host cell and host species.
Transmission of Viral Diseases
- Aphids (greenfly and/or blackfly) are the most common and important vectors of
plant viruses. Most insect vectors have sucking mouthparts, used for feeding on
plant sap. A vector, or “carrier”, is an organism that carries a disease producing
microorganism from one host to another. - Eelworms (nematodes) have also been shown to spread some soil-borne virus
diseases that damage Irish potatoes, strawberries and other crops, including barley