forgive if this a stupid question but if you save your own seed every year arent the chances of disease more apparent
Relatively few plant diseases are spread by seed, and you don't save seed from diseased plants anyway if you can avoid it, so the risk is quite small. And in any case, buying in seed risks bringing in new disease, just as much as saving seed risks perpetuating existing disease.
or is it something else like weaker crops or something to do with cross pollination? :blink:
Um. F1 hybrids don't come true from seed, but traditional (open-pollinated) types are required to by law. So, so long as you don't cross two different varieties, you can maintain a strain just as well and as stably as any specialist (and possibly better than some because you're paying more attention on a smaller scale). Also, any gradual change in the selection will be towards a plant that's more suited to your particular conditions, so you may actually end up with a better, more suitable strain than the one you originally bought.
If you cross two strains you create your own F1 hybrid, which may be brilliant or may be dodo, and then you have to either recreate the cross each year (which means you need to maintain both parent strains as well), or attempt to stabilise a new true-breeding strain, which is a PITA and a long, slow process!
Bottom line is that, if you do a good job, you can get just as good (or better) seed from saving your own as from buying in. The drawbacks are the amount of work needed (which is less than you might think, but still more than writing a cheque and filling in an order form), the difficulty of creating new varieties (most people eventually succumb and try their hand at breeding, but worthwhile new varieties almost always have to be bought in), and the impossibility of maintaining an F1 strain at home (if your favourite variety is an F1, you're stuffed). The biggest benefit is cost saving; also, you can maintain rare varieties that the seedhouses don't stock for one reason or another. Finally, there are ecological benefits to keeping as much genetic diversity as possible, including different selections of the same nominal variety.