Nevermind

Dear Tai-Guo

You're right, intermediate infection types are important in rating resistance to rust ( and powdery mildew) diseases of plants. Many major genes do not give the complete, hypersensitive type of resistance commonly associated with major genes, and ITs can also vary  depending on plant state for temperature- or age-sensitive genes.

The best discrimination I know of is visual. On your right panel, all rust pustules are perfectly orange and clear, as in normal susceptible plants, whereas a number of them are brown on the left panel - which might be evidence for some necrosis and some level of resistance. You can go back to the original descriptions of ITs in wheat rust diseases in the 'classic' work of the Cereal Rust Laboratory in St Paul, Minn, USA, back in the 1960s and 1970s, for a description of these types. You can also contact current rust workers, such as Dr Kolmer, for more explanations.

If you have the opportunity to do so with some of your plants, you can also harvest the spores from the lesions, count them and test their infection efficacy ( by re inoculating known quantities of spores onto susceptible plants and measuring the number of daughter lesions). This will give you an estimate of the level of resistance associated with intermediate ITs relative to the fully susceptible one, and hence a way of deciding whether it's important or not to take these intermediate ITs into account for your own work purposes.

Hope this helps

Didier

Thanks a lot, it is helpful for understanding the intermediate and susceptible infection types. In my opinion, the ITs in the left panel showed intermediate response, and the right susceptible. But some other scientists suggested that the are spores showing and can spread to other plants, even if some spores are getting to be dead (brown color) and the plant tissues do not shown necrotic or chlorate response.  

Hello again,

I agree with your classification.

It's worthwhile to remember that in rusts ( and powdery mildews), IT determination is based on epidemic monocycles ( i.e. one pathogen generation, from  pathogen inoculation to the production of the new spores/offspring), whereas quantitative resistance in the field is the result of polycyclic infections. Therefore, such resistance do not require complete suppression of spore production to be effective. Intermediate ITs may not necessarily lead to no spores being produced at all, but can result in fewer spores or less infective ones, sufficient to result in slower epidemics ( and thus quantitative or 'slow-rusting' resistance) in the polycyclic processes of field epidemics. This is why I suggested counting the spores and testing their infection effcacy to decide on the status/effect of such intermediate ITs in your case.

Best

Didier