[rufty]

Hi,
I was lost in thought on the way to work and a thought occurred: In a standard crop rotation you might have three plots in a row and (for example) have peas in the centre plot year 1 then move them to one of the outside plots in year 2.
So far so good, but one of the reasons I believe you practice crop rotation is to mean that diseases don't accumulate. However if you had a pea disease in year 1 on the centre plot then in year 2 assuming the peas had a healthy root system would they not have some roots heading from the outside plot into the old plot and therefore pick up and re-propagate the disease from the centre plot? I suppose it all depends on how big the plant in question's root system is and on the spacing between your plots, but I've never seen any guidance on plot spacing based on this concern. I suppose to prevent this you'd have to space your plots at least twice the distance of any one plant's root system.
Yes I'm probably worrying about nothing but with the plot settling down for its winter sleep I thought I'd share my thoughts ;-)

[BabbyAnn]

if you had a pea disease in year 1 on the centre plot then in year 2 assuming the peas had a healthy root system would they not have some roots heading from the outside plot into the old plot

Not sure what peas you are growing or what size plots (beds?), but pea roots are generally not that big.  But I think I understand what you are trying to say about the proximity of beds to each other and how year one diseases may affect year 2 on another bed.

I think of all the plant diseases that you need worry about year in year out, white onion rot and clubroot are the worst which target onions and brassicas.  Once these diseases are established on your plot, then proximity of beds don't really come into it - there is no known cure, so you accept it and deal with them.  With clubroot, go for resistant varieties and raise the soil pH with garden lime - it's not a cure but still enables you to still successfully grow brassicas.  White onion rot on the other hand is bad news - it's a bit of a lottery if you do plant alliums whether it will strike and sometimes not worth growing.

Other plant diseases tend to be more of a fungal/mould which are seasonal and weather related for example powdery mildew.  The spores are in the air so may land anywhere so in terms of crop rotation, I wouldn't worry about mildew.  

[mumofstig]

You can only rotate as best you can and, specially on a small plot, keep your fingers crossed

When allotment plots were full size, I spose it was easier to use wide spacings, but nowadays we like to use every square inch to grow in.

I've got onion white rot on one third of my plot   but don't suppose it will be long before it has spread all over, cos I'm not disinfecting my feet everytime I move from one part to the other