There are 2 methods you can follow for inoculating seedlings with spore suspension

1. Mix the spore suspension in low temp molten agarose and spot on the leaves/stem

2. Spray the suspension on seedlings and cover with polypropylene transparent bags. Make small holes in the bag by pricking with needle and also spray sterile distilled water in the inner side of the bag to maintain moisture and facilitate infection.

For infection with sclerotia, wrap the sclerotia in wet cotton and put it between the stem and branching 

Hello Héctor Andrés Ramírez

I can advise you for foliar fungal plant pathogens (powdery mildew fungi), please have a look to the following references

With best regards

Jawad

Hi Hector: which diseases you want to work with , every disease need special conditions of infection. 

Hi oadi. I'll work with Peronospora variabilis on quinoa

I once used cottonbuds (for cleaning your ear) which you dip in spore suspension and streak onto leaves. We used it for Z. tricitic on durum wheat (long leaves) so maybe its not the best for your experiment, but very low cost. 

Hi Hector.

How you inoculate a plant will depend a lot on the type of pathogen you are dealing with.  If it is a fungus, you have to find out the best conditions it grows best in and you have to create a microclimate necessary for infection to take place.  For most foliar fungal pathogens, seedlings can be inoculated with spore suspension by spraying it on the leaves till run off.  Then cover the inoculated seedlings with clear polythene bags for 48 hours (make sure you mist the inside of the p/bags first with sterile distilled water before covering the inoculated plants).

Hope this helps.

I routinely make wheat stem rust fungus inoculations on wheat seedlings. Because of the issue of galsshouse space I stunt my pants 6 days after planting with 50 ml of Maleic hydrazide (was used a while ago as a herbicide) at a concentration of 0.2 g/L which pour into 9x9x12cm pots which contain 8 seedlings each. The plants become stunted and have thick dark green leaves which are ideal for the fungus to feed on!! Also the big advantage is growing several plants on a small area.  tomato you will have to determine whether maleic hydrazide works on tomato and the ideal concentration of maleic hydrazide to use. If you're working on plant resistance to pathogens then this method is only effective for those genes that function from seedling stage and not just at adult stage. 

I use an airbrush and compressor (about $6 per brush and $50 for the compressor) for inoculations. Great for speed and uniform distribution of inoculum on the leaves. You can use low inoculum concentrations to get well spaced infection sites.

To ensure successful inoculation keep your inoculated plants for a couple of hours (13 to 24 h) in a very humid environment. Put your plants in plastic bags that have a small amount of water at the bottom and seal the open end. This works really well. The film of water that develops on the leaves is essential for both fungus and bacteria inoculum activation, survival and penetration.

After misting, to avoid cross contamination in a room where there is work on other pathogens or different isolates of the same pathogen, I use cellulose cross bottom bags (about $100/ 1000 bags) to cover the post and use an elastic band to hold the open end of the bag to the pot. You can have them custom made to suit the size of the pots you use. The bas are also handy in that they can be tipped over for spore collection.

I have attached a picture that shows what the setup is like.

Wish you all the best in our work.

Since you try to inoculate your plants with downy mildew pathogen... the relative humidity on the leaves (Film of water) is the most critical factor .. wet the leaves.. and inoculate them either by touching infected leaves with your plant, or brushing, or spore suspension .. You have to cover the inoculated plants for 16-24 hrs ... search for yellowing on the  leaf adaxial   and mildew growth on the leaf abaxile ... Read this will help u" First Report of Quinoa Downy Mildew Caused by Peronospora ." 

Dear Hector>>> This Synopsis will help u 

APS > Publications > Plant Disease > 2012 > January

First Report of Quinoa Downy Mildew Caused by Peronospora variabilis in the United States

January 2012, Volume 96, Number 1

Pages  146.3 - 146.3 

A. L. Testen, Department of Plant Pathology, The Pennsylvania State University, University Park, 16802; J. M. McKemy, USDA-APHIS-PPQ-National Identification Services, Beltsville, MD 20705; and P. A. Backman, Department of Plant Pathology, The Pennsylvania State University, University Park, 16802 

Go to article: http://dx.doi.org/10.1094/PDIS-09-11-0728 

 Quinoa is plagued by downy mildew caused by Peronospora variabilis Gäum (formerly Peronospora farinosa f. sp. chenopodii Byford) (1). Quinoa production has spread beyond native Andean ranges and quinoa downy mildew has been reported in India, Canada, and Denmark (1). During the summer of 2011, quinoa trials were established to determine the ability of quinoa to grow under Mid-Atlantic conditions and monitor for regional disease problems. In July, after cool, rainy conditions, downy mildew-like symptoms were observed on quinoa at research plots in Centre and Lancaster counties of Pennsylvania. Symptoms and signs consisted of irregularly shaped areas of foliar chlorosis or pink discoloration accompanied by dense, gray sporulation on both leaf surfaces. Sporangia were tan to gray-brown, semi-ovoid, often with a pedicel, mean length of 31 μm, and mean width of 23 μm. Sporangiophores branched dichotomously, and the terminal branchlets curved and tapered to a point. Orange oospores were present in field samples of leaf tissue. DNA was extracted from infected foliar tissue and sporangial suspensions. A seminested PCR protocol (2) was used to obtain partial internal transcribed spacer (ITS) sequences of six Peronospora isolates. The sequences shared 99% maximum identity to a known P. variabilisaccession (FM863721.2) in GenBank. A voucher specimen was deposited into the U.S. National Fungus Collections (BPI 882064). Pathogenicity of each of two strains of P. variabilis was confirmed by inoculating quinoa with sporangia (4). Sporangia were shaken from leaves in sterile distilled water and the suspension was filtered through cheesecloth. A 0.01% Tween solution was added and the suspension diluted to 103 sporangia/ml. With an atomizer, a 10-ml sporangial suspension (or sterile water for noninoculated control plants) was sprayed onto one flat of 18 2-week-old quinoa plants, and relative humidity was increased to saturation using a humidity dome for 24 h. After 1 week, chlorosis and pink discoloration were noted on leaves of inoculated quinoa, and after 18 h of subsequent increased humidity (>95% relative humidity), dense gray sporulation was observed. No symptoms were noted on noninoculated control plants. Sporangia and sporangiophores were examined morphologically and confirmed to be P. variabilis, confirming Koch's postulates. For culture maintenance, 2-week-old quinoa leaves were placed onto a sporangial suspension on top of 1% water agar and maintained in a growth chamber at 20°C with 16 h of light per day. Quinoa downy mildew is seedborne (3) and initial infections may have occurred from oospores in the pericarp, despite intensive processing of consumable quinoa seeds to remove saponins. To our knowledge, this is the first report of quinoa downy mildew in the United States and also the first report of P. variabilis in the United States.

References: (1) Y. Choi et al. Mycopathologia 169:403, 2010. (2) D. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) S. Danielson et al. Seed Sci. Technol. 32:91, 2004. (4) J. Ochoa et al. Plant Pathol. 48:425, 1999.

Hi Héctor

Some references 1

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwjojYKQyd7XAhViMJoKHX4cBwcQFggxMAE&url=http%3A%2F%2Feuroblight.net%2Ffileadmin%2Feuroblight%2FAlternaria%2FProtocols%2FArtifical_inoculation_Stammler.pdf&usg=AOvVaw2f3LjtVzecuENd57AN28k7

Dear Mr. Kangara

Thank u for the nice note on inoculation procedure For evaluation of wheat seedling -rust pathogen response( Evaluation the seedling resistance) , I would like u to use this procedure ... .... in 1990s , I developed a very good procedure to check the both the pathogenicity & diseases response of wheat varieties toward Urocystis agropyri on Wheat seedlings in just three weeks.. by cutting half of the First leaf emerged from inoculated seed to accelerate the the 2nd leaf... then u cut the 2nd leaf to get the 3rd leaf in a short time ...Flag smut symptoms will express on the third leaf in Highly susceptible wheat genotypes while the the smut sori will appear on the 4th leaf for MR , 5th leaf on the MR while their is no sori at all or very light infection on the 6th leaf for R or HR . So u can use this technique for rust test.. by inoculating the seedling at i 3d leaf stage ...