Stem rust, stripe rust and leaf rust, have periodically each caused yield losses in wheat crops that exceed 50 per cent. In very severe situations yield and quality losses have even been higher.
Prior to the 1960s stem rust was such a serious disease many northern farmers wouldn't grow wheat because the risk was so high.
Plant breeders have played a major role in developing varieties resistant to each of these rust diseases. However, it has been a very difficult task combining good resistance to each of them in the one variety, especially in combination with all other variety requirements, such as high yield, other disease resistances and high quality. Evolving new rust strains constantly adds to the task of maintaining good resistance as a resistant variety today can be a susceptible one tomorrow.
Most varieties grown today will have good resistance to at least one of the three rusts but there are several popular varieties, as well as less popular but still important varieties, that do not have strong resistance to all three of the rusts.
Therefore the importance of knowing the rust resistance status of all varieties grown on a property, as well as keeping up to date on any changes in resistance status. A given variety's status sometimes changes depending on the development of new rust strains (common to all three rust types).
Identifying what rust or other diseases one has and appreciating environment conditions that favour a given disease development (each rust has different temperature triggers for example) is important. Also where one is growing a variety with susceptible status to one of the rusts, or other disease like yellow leaf spot or Septoria, having a planned fungicide program should favourable climatic conditions occur is also important.
Stem rust, large dark reddish-brown pustules on stems, both sides of leaves, leaf sheaths or heads, is favoured by warm humid days, generally when temperatures are 15 to 30C. This has historically been the worst of the three rusts and best protection is via growing resistant varieties. Foliar fungicide control is worth considering if applied early to a developing infection on a susceptible variety.
Stripe rust, initially identified by yellow pustules, more infects leaves and is triggered by temperatures in the range eight - 20C and when conditions are moist. Some resistant varieties are susceptible in the seedling stage, hence knowledge of a variety's resistance rating is important for deciding on fungicide management. Fungicide control, if early in an infection, can be effective and profitable.
Leaf rust, identified by smaller oval pustules, mainly on the upper surface of leaves, is perhaps the sleeper rust diseases as more varieties tend to be released with lower resistance status. However in favourable conditions yield loss in susceptible varieties can be severe. Moist conditions when temperatures are in the range of 15 - 22C favour the disease.
Yellow leaf spot, in a moist late winter has occasionally caused over 50 percent yield loss in susceptible varieties. Symptoms tend to be tan spots with yellow margins. It can start in seedlings spreading to upper leaves as plants run to head. Wheat on wheat tends to favour the disease (unlike rusts) but in a bad year rotation sometimes don't help infection level. Some varieties have useful tolerance and fungicide treatments, if timely, can help contain the disease.
Septoria blotch, again a sometimes devastating disease, especially in southern environments, can also be protected by growing resistant varieties. Symptoms include irregular shaped tan to brown blotches on leaves with tiny black specks within the blotches. Like yellow leaf spot the fungus can last on stubble from a previous wheat crop. Fungicide treatment can be a feasible option.
Various information sources can help with detailing variety susceptibility to crop diseases including the NSW DPI Winter Crop Variety Sowing Guide or the GRDC web site. Being prepared for various possible disease threats, especially if a wet year develops, is the starting point for avoiding what can sometimes be a costly disease outbreak.
Next week. Central west research shows summer fallow cover crop can be profitable.
Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.