Pasture Diseases – Crown Rust & Root Rot

Crown rust or Puccinia Coronata is the major fungal disease that occurs in perennial ryegrass.

It leads to a 37% DM yield loss and 94% loss in green matter. In New Zealand, green matter losses of up to 53% have been recorded. This has lead to clover dominance in mixed ryegrass/clover swards, reductions in root weight, tiller number and leaf areas, all of which have been linked to disease severity.

In this article, we’ll be discussing crown rust & root rot, and what we can do to minimize the risk of infection.

How to spot crown rust:

Crown rust is usually found in perennial ryegrass and tall fescue. If you happen to notice bright orange powdery spots (also known as pustules) on the leaves, chances are, your ryegrass has contracted crown rust.

The first signs are small and round, orange-yellow pustules that appear on both sides of the leaves. Afterwards, the pustules join together to form evenly affected areas. Small and black pustules form later in the season.

The pustules produce orange spores that are carried by wind and rain to the other leaves. These pustules germinate to produce new pustules.

20 degrees Celsius with high humidity is the ideal climate for these pesky pustules to thrive!

Prevention of crown rust:

You can prevent crown rust from taking over your pastures by using more resistant cultivars in the areas where crown rust occurs.

Resistant cultivars are very much required especially due to the tendency to reduce the amount of nitrogen used. The lesser nitrogen is applied, the more that crown rust can affect ryegrass stands.

You can also avoid rust by grazing pastures at the right time.

Applying N fertiliser also helps in preventing rust. If rust is already present, simply hard graze to remove the infected pasture and apply N. If done rightly, the new growth will show very little rust.

Fungal Pathogens:

Apart from the awful effects on foliage, fungal pathogens can also infect the inflorescence, thereby altering the seed yield. The pathogens that cause these are stem rust, scientifically known as Puccinia graminis and blind seed disease, also known as Gloeotinia granigena. These two fungal pathogens show up on the seed heads on a varied range of pastures.

Both these fungal pathogens are the primary cause of herbage and seed yield losses in perennial grasses in Australia and New Zealand. Farmers are taking a lot more effort to screen perennial ryegrass for resistance to these diseases.

However, the progress in resistance breeding using recurrent selection has been very slow because of the huge gaps in genetic knowledge.

Studying the genetic resistance to this disease is the major focus of ryegrass breeding programs. Breeders identify genotypes that carry resistance genes for crown rust after natural or artificial infection.

Root Rot in Subterranean Clover

Subterranean clover, an important part of Mediterranean dryland pasture ecosystems, is largely used as a winter annual pasture. It is mostly found in the south-west of Western Australia. Their shallow root system helps easily build up soil nitrogen, organic matter and physical structure.

It increases the stock-carrying capacity and also helps increase the yield in successive cereal crops because they can fix nitrogen. Its other pros include the capabilities to endure different pasture management practices and its ability to prevent soil erosion and it’s tolerance to water-logging.

Subterranean clover is extremely vulnerable to root rot or damaged roots. The main cause of root rot are fungal pathogens and oomycete.

The decline in quality of pasture due to root disease was first observed in Western Australia in the late 1960s. Root diseases are a continuous reason for the decline of pasture in Australia, also causing a reduction in seedling establishment and pasture productivity in annual legumes.

In areas prone to diseases, extremely bad cases have caused up to a 90% failure in seedling emergence.

An important finding to note is that root disease has mainly been limited to the early part of the growing season.

Soil-borne fungal pathogens love to make themselves a comfortable and cosy little home on subterranean clover pastures because it’s easy for them to survive on the infested residues that crop up during dry summer periods.

The soils in such regions are already low on nutrients and so, there’s no competition from other microbes, giving the fungal pathogens full control over the pastures.

How to spot root rot

Since root rot takes place below the soil, it often happens that the disease goes unnoticed until it gets serious.

Root rot can be spotted by studying the roots that will turn soft and brown. A healthy plant has firm and white roots. However, when the soil is soggy, the fungal spores multiply in number and the fungus starts to spread. As the fungus slowly takes over, the healthy parts of the root turn brown and mushy.

Apart from the usual herbage and seed losses, fungal pathogens also cause a change in botanical composition and feed quality also gets affected.

Sometimes, the damage done by these fungal pathogens to this self-seeding pasture legume is so bad that reseeding may have to be done.

Not to be a name-dropper, but these are the names of some of the most common pathogens that put our precious pastures in grave danger – phytophthora clandestinea, pythium irregular, aphanomyces, rhizoctonia solani, phoma medicaginis, pratylenchus, trichodorus and radopholus.

It usually causes two types of damages:

1. Damage to the hypocotyl, cotyledons and root radicle – this damage happens when the seed is germinating. This, the seeds die before emergence and this is called ‘damping off’.
2. Seedlings have damaged roots

When we say root damage it could mean a minor shortening of the lateral roots to the loss of the entire taproot.

The root pathogens all work together as a team and thus, it is a challenge to source genotypes that show resistance to multiple pathogens. Apart from plant nutrition, soil moisture and soil temperature also have a huge effect on just how dangerously the disease will spread.

Challenges in disease management

Fallowing the area before cultivation and reseeding still did not lower the levels of root rot remaining.

And while the productivity of pastures has definitely improved with the use of fertilisers, this has caused an increase in soil acidity problems thus affecting plant growth and making them more vulnerable to attack from plant pathogens.

As mentioned earlier, another huge challenge is to locate host genotypes with resistance to multiple pathogens since root pathogens tend to work together as a team. In addition, soil moisture and soil temperature actually help these root pathogens work together more effectively, and cause even more damage!

Opportunities in disease management

Fungicides like metalaxyl, thiram and propamocarb have shown a lot of promise in treating root rot in subterranean clover. Metalaxyl showed the best results while the other two, thiram and propamocarb were a little less effective.

Manipulation of soil pH by using lime can also be explored as a way to combat root disease.

Today, one of the best methods for disease control that has shown a lot of promise is the use of subterranean clover cultivars that have an increased field resistance to root diseases.

The Mediterranean region is a great source of host germplasm that have very good resistance on one or more foliar and soil-borne necrotrophic pathogens. Germplasm from this area is used in the search for new cultivars of subterranean clover that will have improved resistance to soil-borne fungal and nematode diseases.

Hope you enjoyed this article, and remember to leave a comment below and we promise to reply.

Happy Farming!

- The Dedicated Team of Pasture.io, 2020-09-22