In this case you can only use DNA analysis. or wait till fruitification. Conidiation is very similar in many powdery mildew species

You might refer to the APS Education Center's activity on powdery mildew identification - it contains a key to morphological characteristics. 

http://www.apsnet.org/EDCENTER/INTROPP/LABEXERCISES/Pages/PowderyMildew.aspx

There are several books and journal articles, like those of Uwe Braun, a world authority in Erysiphales, where you can find morpholigical data of powdery mildews. However the first thing you need to know is the host species identity because usually powdery mildews are very specific and have few morpholigical traits that allow its identification without know its host species previously.

Hi Dear Dr.Petkam

I think you ask about morphological charactristic of powdery mildew species? am I right?

based on conidia and conidiphore shape you can categorized in 3 groups (Anamorph stage)

1- if conidia shape cylindrical or barrel shape that settle on top of conidiphore as a chain ( this form called Oidium)

2- if conidia shape as a missile or rocket that settle on top of conidiphore as 1/3 of biggest thickness rest on the top of conidia ( called Ovulariopsis)

3- if conidiophore produced different conidia shape ( called oidiopsis)

if you prepare a slide from different stages of Anamorph ,it is better to take a piece of tape and slowly return on conidial stages and take it slowly and then observed on microscope and then you can look the chain of conidia and other characteristics

Meanwhile if you identified anamorph stages that may be teleomorph stages related to one of these speiecs

1- if it is oidium  that teleomorph  may be Erysiphe,Podosphera, Microspheara, sphearotheca,Uncinulla

2- if it is Ovulariopsis that Tele... is Phyllactinia

3- if it is Oidiopsis that tele... is leviellula

Yes it is difficult to distinguish conidia morphology between species causing mildew especially, they are for most elliptical to ovoid. However, I could tell you to observe the basal cell conidiophore that is different from one species to another. For example, for Erysiphe graminis (Blumeria graminis) cereal powdery mildew agent, basal cell is wide. However, it is twisted at Erysiphe necator, the pathogen of grape powdery mildew

thank you very much for your inputs ... as for the answer of Dr. Karimi the specimen with me must be Ovulariopsis sp. I will look for teleomorphic stages...

regards

Srinivas

this one looks like a Pseudoidium anamoph. Pseudoidiums have Erysiphe as teleomorph (when known). But without knowing the host plant species, without the teleomorphic properties and measurements of conidial size, conidiophore dimensions it is impossible to identify it.

with my experience on powdery mildew, you may measure length and widths of conidia. then refer Uwe's monograph. however the best quick method I would recommend is use both morphology and molecular data. It is east to amplify ITS on powdery mildews. good luck

Based on my experience with cereal mildew (Blumeria..), even conidia of the same forma specialis may look slightly different, depending on the growing conditions of the colony (laboratory, field), on the colony age and nutritional status of the leaf etc. The differences are mainly in size and elongation. Keep in  mind, that conidia properties evolved by natural selection for optimal spread from their hosts. Conidia of different mildews might therefore look similar, with overlapping random variability (so as compact cars from different producers look similar). It is safer to use DNA (as answered by Maria Kozlova) or use test infections.

Dr. Mahmoud Reza Karimi  has explain really very easily, nicely.  

The differences among  powdery mildew fungi are summarized in the following diagram

Good luck

I recommend Jawad's answer

Best Regards

I agree with Majida Hafidi