Factsheet: Tackling wheat leaf rust– Kenya’s most devastating wheat menace

Affecting seven in ten wheat growers and causing up to 100 per cent in losses, wheat leaf rust is the single most devastating disease or pest for wheat growers across Kenya.

“I’m just from looking at ten blocks of land and have seen an emergence of wheat rust in eight of them. It is near impossible for any wheat farmer to bring his crop to term without experiencing instances of wheat rust,” informed Joseph Wangai, an agronomist and wheat farmer for over ten years.

It is esspecially common in low altitude regions of Laikipia, Nyeri, Nakuru, Nyandarua and parts of Uasin Ngishu.

High cost of control

Wheat leaf rust costs about Sh7,000 per hectare to manage and according to Joseph, this rises to about Sh10,000- 12,000 per hectare when the infection rate is above 20 per cent. This is exclusive of the cost of hiring a boom spray and labour.

What makes the fungal disease particularly problematic for farmers is it’s mode of attack which makes it hard for them to identify and arrest it on time. 

It first affects the bottom of the crop and underneath the leaf before creeping all the way to the head. This makes it difficult for farmers who scout their crop from a distance to realise and control it on time.

Scouting for leaf rust

A wheat farmer will need to get into the shamba and inspect not less than eight to ten positions on the farm, especially positions on higher ground. “In gentle slopping land, raised regions of the farm are hotspots and the first area of attack for wheat leaf rust,” Joseph added.

Signs of wheat leaf rust

1. Brown spots on the surface of the leaf and stem
2. Head smut/ bleached head. Instead of maturing from green to brown, the wheat heads start turning cream white

Key times of attack

1. Abdrubt changes in weather- either a shift from excess heat to wet humid weather or hot humid weather to wet dry weather

“If it rains and at the same time there is excess sunlight (heat) during the day, this provides ideal hot and humid weather conditions that accelate the spread of diseased spores. Continuous rain with cloud cover and no high diurnal range of temperature leads to low prevalence of the disease,”

Farmers should ideally apply a preventive cover after the rains to avoid disease outbreak.

2. When the crop is weak/its immune system is compromised as it is expending most of its energy in carrying out other processes. 

• During head formation, the crop’s immunity is low because all the amino acids, vitamins, and sugars are concentrated on booting
• Milky stage: After the pannicle has formed the number of rows per head is countable and you can calculate the number of grains. 

3. Volunteer crops- If you had previously planted wheat on the same plot, this (volunteer crop) germinates earlier than the currently established wheat crop and are ideal vectors for the disease.

“If your crop is attacked at the booting stage you will immediately notice a prevalence of wheat rust pores and if not controlled both the grain and the stalk do not yield. Your grain will be light and the yield could be cut by up to 80 per cent and at times we see absolute losses,” the agronomist explained.

Dangers of reusing seeds

Reusing seeds already infected by leaf rust almost guarantees that your new crop will be affected by the fungal disease. It is therefore advisable that farmers grow clean Kenya Plant Health Inspectorate Service (KEPHIS) approved seeds. 

“This will necessitate carrying out three to four rounds of spraying which eat into your entire profits,”

Preventing Leaf Rust

Prevention, as it’s been said is better than cure thus making it the most cost effective method of dealing with wheat leaf rust.

It involves:

1. First spray at tillering
2. Second spray at beginning of booting/ head formation
3. Third spray at the mid-head formation stages

If the previously discussed weather patterns are unfavorable you will unfortunately have to conduct another round of spraying. Farmers should also be keen to observe the outlined dosage rate.

However, if conditions are favorable, i,e, it rains throughout the growth stages of the crop or the wheat is grown in areas less susceptible to leaf rust, a farmer can get away with one or two sprays.

“Most products in the market for prevention purposes are one liter per hectare. This means that regardless of the amount of water you use, the entire liter should be used in 2.5 acres,” he said.

If you ignore the dosage rates this will build the disease’s resistance increasing its virulence.

For farmers spraying their crop during the rainy season they will have to adjuvants– products added to increase agrochemicals effectiveness– to their fungicide before spraying.. These are either ionic spreaders like Aqua Wet or organosilicon spreaders like Magnum Gold.  

“Adjuvants also referred to as wetters, spreaders or stickers contain sparse droplets that stick and spread on the surface of the leaf. Even if it rains, the fungicide droplets will be stuck on the surface of the wheat and cannot be washed away. Most fungicides are also systemic and therefore get into the membrane of the leaf quickly. In about 30 minutes to two hours they would have absorbed 80 per cent of the fungicide,” illuminated Wangai.

Ionic spreaders and wetters are relatively affordable costing between Sh600-800 per liter. One liter can handle up to six hectares (10-12.5 acres). A liter of organosilicon ranges from between Sh1,700 to Sh2,200 and covers 20-30 acres.

Adopting tolerant varieties 

These are varieties adapted to lower altitude areas that often require just two sprays.

• Kasuko- It is the most tolerant as it grows faster than other varieties such as Njoro II. This means a farmer will have to only spray it twice. It is also adapted to lower rainfall, but its yields are relatively lower.
• Hawk
• Planet

Controlling Leaf Rust

This involves the use of triazole fungicides that exist as single molecules or a combination of two to three molecules. It is important to inquire about the agrovet you are buying from whether the fungicide you are purchasing is curative, preventative, or serves both purposes. 

Common triazole molecules are Tebuconazole (more preventative) and Flutriafol (curative and preventive).

Examples of brands available in Kenya:

1. Bayer- Folicur (Tebuconazole)
2. Amiran- Orius (Tebuconazole)
3. Jojemi- Dokta Cure (Tebuconazole)
4. Advanta, and Adama, have combinations of both curative and preventative triazoles.

“Farmers in coffee/ sugarcane growing regions sometimes apply chemicals used to combat rust in these crops on their wheat. This is ineffective as the fungicides are only effective on the particular crop they are designed for,” the expert cautioned.