Fungicides basics – working out what your crop needs

Most of us know fungicides by their brand names. But when you want to get scientific about farming, it’s important to understand a bit more about which ones to use in different crops, and why they suit particular situations.


These represent the largest and most significant group, and include active ingredients such as tebuconazole and cyproconazole. These tend not to be the most persistent chemicals in terms of how long they protect the plant, but they do offer both curative and protectant activity on most disease types in many crops.

Triazoles also tend to be a lower disease resistance risk, so are commonly used alone. Many are also off-patent, which means that generic copies have lowered the price of many of these products. Big differences exist in their activity however; prothioconazole is the most recent of the triazoles and is an excellent product in cereals and canola, but offers far less activity on pulse and maize diseases.


The strobilurin family of chemicals are highly protectant, and when they were discovered in the late 1990’s, made a very dramatic contribution to disease control. Not only in cereals crops, but azoxystrobin, picoxystrobin and pyraclostrobin have been used in maize, fruits and pulses. The particular strength is the duration of control that they provide against initial infection, partly due to their highly systemic activity which allows them to protect new growth.

The chart shows a trial in maize that I carried out in both a dry season and a wet season in England; there was no yield response in the dry year, but the previous very wet year saw a yield increase of around 10% from two different fungicides I trialled.

Resistance is widespread in many cereal diseases and developed very rapidly. They are still highly effective against other diseases however, such as Maize Rust, and they do provide physiological benefits to plants (they improve nitrate reductase activity for example, so can increase yield even in the absence of disease).


SDHI fungicides have not arrived in Kenya on field crops yet, but have been a major breakthrough in other parts of the world. Similar in some respects to strobilurins, they offer excellent protection and eradicant activity on several key disease such as Net Blotch in Barley. They tend to be very mobile in the leaf, and show strong systemic activity.

Because they work on just a single site of the fungi, they are at a high risk of resistance developing if they are not used with other active ingredients in a disease control program. Resistance has been identified in the field at low levels already, hence why manufacturers tend to only offer them in mixtures with other chemicals such as triazoles.

I believe the SDHIs could be transformational to barley disease control in particular, provided there activity is safeguarded with anti-resistance measures. Let’s hope that some of these chemicals are approved for use here very soon.


Chlorothalonil remains a vitally important chemical, despite having been around for several decades. Purely a protectant, it has no activity on any infection already in the plant so must be applied before disease appears. It is highly useful in Pulses, and in wheat for protecting against wet weather diseases such as Septoria.

It has no movement in the leaf so only protects what it hits, and it can antagonise some chemicals when tank mixed, so be advised to check with the supplier or agronomist especially when mixing with micronutrients. In purely eradicant situations where you are fighting a well established disease, there is evidence that chlorothalonil can reduce the performance of some triazoles, so its use should be avoided.

Mancozeb is a similar chemical to chlorothalonil, although is more effective on potatoes and horticultural crops, and rarely used on cereals and maize.

Carbendazim is still widely used in many crops, and is a low cost, effective protectant fungicide with activity on stem based diseases in many cereal crops. It is worth noting however that it has been found to be toxic to Earthworms. Sclerotinia populations often show reduced sensitivity to this chemical too.

Useful resources:

The below table is an example of the ratings that the AHDB give each fungicide active ingredient against a range of foliar diseases:

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