Adult plant resistance and disease ratings explained

Hugh Wallwork, SARDI

The expression of resistance to a pathogen varies between host varieties and different diseases. Many different descriptive terms have been used to try and categorise these differences but the most useful, albeit imperfect ones, which have been used in Australian cereal pathology have been seedling resistance and adult plant resistance. The following descriptions provide a useful guide to these two terms:

Seedling resistance
This is resistance that is effective at the seedling stage and then normally throughout a plant’s life cycle. It is almost always controlled by a single ‘major’ gene that provides very strong resistance which, in most cases we call R or R-MR. These genes have mostly been identified and given an identifying number and have been preferred by plant breeders for controlling the rusts due their ease of use. They have the disadvantage that once the pathogen overcomes this type of resistance the variety often becomes completely susceptible. Most of these genes are quite prone to resistance breakdown, particularly if inoculum levels are high. Examples are the Yr17, Lr37 and Sr38 genes controlling stripe, leaf and stem rust respectively that are all located on the same VPM segment of chromosome. Each of these genes has become ineffective since Trident, Camm and other varieties carrying them were released.

Adult plant resistance (APR)
This form of resistance is not effective at the seedling stage but increases in effectiveness as the plant develops. It is typically controlled by one or more ‘minor’ genes. Individually, these minor genes are much less effective than major genes, but a number of minor genes when combined can provide more effective and often durable resistance. The level of resistance provided by minor genes is very variable, will start to be expressed at different growth stages and is more likely to be affected by environmental factors such as temperature and plant stress. Unlike major seedling resistance genes only a few minor genes have been identified and numbered.

Exceptions to the above general rules are mostly found with yellow leaf spot in wheat and the spot form of net blotch (SFNB) in barley. Resistance to these diseases in South Australian grown varieties is mostly expressed in the seedling stage but is controlled by minor genes. Although there is generally a good correlation between resistance in the seedling and adult plant growth stages for these diseases some genes are only effective in the seedling and some only in the adult plant growth stages so care needs to be taken when extrapolating between the two stages.

When looking across the other different fungal pathogens and different wheat and barley varieties we find that, with the exception of the rust diseases, most resistance is APR. Some resistances to scald, net form net blotch and powdery mildew in barley and septoria tritici blotch in wheat are also controlled by seedling resistances, but these have not been used to any great extent in South Australian breeding programs and where they have they have not lasted for long.

Variation in expression of adult plant resistance
Minor genes for APR will be working through different mechanisms in the plant. Some will be reducing the proportion of spores that are able to infect the plant, so reducing infection frequency, whilst others will be affecting the growth of the fungus in the plant. This latter type of resistance may lead to fungal lesions having a lot of yellowing (chlorosis) and death of leaf tissue (necrosis) but with few spores being produced whilst other fungal lesions may produce little chlorosis and necrosis.
Genes that slow the rate of fungal growth in the plant cause a longer latent period (time from infection to new spore production) and this can lead to a considerable reduction in the rate of an epidemic. Finally, some resistance genes are affected more by temperature. This is probably also true for other abiotic stresses on plants but published evidence for such differences is poor.

In assessing wheat varieties for APR in the field some of these differences in expression of resistance can be observed. They have not been clearly documented however. This could be a useful new research tool if funding is made available.

Adult plant resistance and problems with variety disease ratings
Because APR is controlled by different minor genes that operate through different mechanisms, the growth stage at which the resistance is expressed and the environmental conditions under which it is best expressed vary from one variety to another. This leads to problems providing a consistent and reliable disease resistance rating. A good example is Wyalkatchem which has been frequently seen in SA to be susceptible or even highly susceptible to stripe rust during August and September whereas at later growth stages and, we think, at higher temperatures, its resistance appears to be much more effective and may be rated as MR-MS or better. These differences partly explain different disease ratings provided by different groups/states. Other varieties such as Janz and Yitpi generally show less marked differences in their resistance over the same period of time.

Because a strategic decision has been made not to inoculate trials early in the season for fear of generating regional rust epidemics, we have been unable to generate comprehensive sets of data clearly demonstrating the different times of onset of APR in new wheat varieties.

SA ratings for adult plant resistance
In SA, disease ratings for the rusts have, where possible, been given to provide growers with appropriate advice for the time when fungicide sprays are required rather than later in the season when APR is more fully expressed. This has the significant effect of also identifying those varieties that are most likely to generate serious epidemics through the production of the most spores early in the season. This in turn helps to reduce the chances of new mutations occurring and further eroding the resistance of our varieties. Some varieties such as Wyalkatchem will therefore appear to have been given too susceptible a rating if observed late in the season.

Durable resistance
Some varieties have resistance that is described as durable because either the variety or genes involved have proven effective in the field when grown over large areas for many years. Durable resistance is usually associated with APR although not exclusively as the Sr2 stem rust resistance gene (in Kukri and Machete) appears to be durable around the world.

Some genes that control APR may lose their effectiveness just as readily as seedling resistance genes. This was clearly demonstrated when Krichauff lost its APR with the arrival of the WA strain of stripe rust. However varieties with good levels of APR will usually have resistance from two or more minor genes and for this reason the resistance provided is likely to be more durable as any changes in the pathogen are likely to result in only stepwise loss of resistance over many years. Varieties with three or more minor genes are ideal and this has been the aim of the CIMMYT wheat program in Mexico in their highly successful breeding for leaf and stripe rust resistance where up to four minor genes have been pyramided into single new varieties.

Contact: Hugh Wallwork
Ph: 08 8303 9382
Email: Wallwork.Hugh@saugov.sa.gov.au