Arable crop diseases and pests have primarily been controlled by fungicides and insecticides. However, due to environmental and safety concerns, marketing constraints and resistance to some chemicals, there has been increasing use of other control methods. These include the use of cultivars that are resistant to or tolerant of some diseases and pests, crop rotation and the adjustment of sowing dates. Biological controls are also being investigated for some diseases and pests.
Some of the major diseases that affect wheat are:
- take-all, which is caused by a fungus that attacks plant roots, and can be controlled by crop rotation
- wheat stripe rust, which is significant in Canterbury, but can be controlled by using fungicides or resistant cultivars
- barley yellow dwarf virus, which is transmitted by aphids
- Septoria tritici blotch, which is the major disease in autumn-sown crops in South Otago and Southland
- fusarium head blight, which is especially problematic in the North Island.
Major barley diseases include:
- scald, caused by a fungus, which is problematic in wet years
- net blotch, which is usually controlled by fungicide seed treatment, though some strains have become resistant to fungicides
- leaf rust, problematic during warm years and in susceptible cultivars, mostly in the South Island
- barley yellow dwarf virus
- spot blotch, which is caused by a fungus, and is a problem in the North Island.
Diseases that can affect peas include:
- pea wilt, caused by Fusarium spp. in soil, which can cause seeds to rot in the ground or the lower leaves to turn yellow, die or have stunted growth
- powdery mildew fungal disease, which forms a white powder over the leaves
- downy mildew (Perosnopora viciae), which occurs in cold wet seasons and causes yellow-brown blotches on leaves and pods
- root rots caused by various fungi, which can cause plants to collapse in hot, dry periods, but may be controlled by fungicide treatment of seed
- viruses – including alfalfa mosaic virus, cucumber mosaic virus, the pea strain of bean yellow mosaic virus – which are usually transmitted by aphids and cannot be controlled, though some cultivars are resistant to some viruses.
The main diseases of maize include:
- northern leaf blight (Setosphaeria turcica) and eyespot (Aurebasidium zeae), which are widespread, and although not generally economically significant, they can reduce crop yields in some years and locations
- stalk and ear rots (Diplodia maydis, Gibberella and Fusarium spp.), and root rot (Rhizoctonia solani), though levels are generally low in most crops
- common rust (Puccinia sorghi), which is occasionally seen in crops but not at levels of economic importance.
Pests that can damage crops or transmit viruses include:
- aphids, which can affect a wide range of crops by sucking the sap, but are more significant as spreaders of viruses that can cause major cereal crop loss
- slugs and snails, which particularly damage seedlings and young plants
- nematodes, which attack the roots of clover and ryegrass plants
- African black beetles and Argentine stem weevils, which can do considerable damage to young maize plants – Argentine stem weevils can also affect ryegrass growth and reduce seed yields
- greasy cutworms (Agrotis ipsilon), which sever young maize shoots at the base of the plant
- wheat bugs (Nysius huttoni), which infests a range of crops – in wheat it pierces the grain and sucks out the nutrients.