Pests enjoy culinary herbs too!

Sean Westerveld, Ginseng and Medicinal Herbs Specialist and Melanie Filotas, IPM Specialist for Specialty Crops, OMAF and MRA

From ONvegetables in The Grower, April 2013

We often hear that most herbs can be used to repel pests in the garden, but herb growers know that they are susceptible to many pests as well. Unfortunately, there is very little information available to herb growers on pests and the strategies that can be used to control them. Just knowing what pests to look out for is half the battle, because it allows for pest control strategies to be implemented before pests become a problem.

Over the past two years, OMAF and MRA staff have been surveying culinary herb crops in southern Ontario to determine the major pests of herbs and eventually develop pest identification resources to assist herb growers. The project was initially funded through the OMAFRA/University of Guelph Undergraduate Student Experiential Learning Program, and summer student Alex Harris was hired to survey herb fields.

There are numerous culinary herbs that can be grown in Ontario, and most of them fall into two families: the mint family (e.g. mint, basil, rosemary, sage, lavender, oregano, thyme, lemon balm) and the carrot family (e.g. parsley, cilantro, dill, fennel, anise, chervil). Chives, tarragon, and fenugreek (methi) are the main herbs not included in these families. Insect pests tend to affect most of the herbs in a certain family, while diseases can be more specific to an individual herb.

Listed below are the major pests identified over the past few years:

INSECTS

Four-Lined Plant Bugs attack most members of the mint family. Since they lay their eggs on perennial plant tissues, they are not usually an issue on annual herbs such as rosemary and basil. Nymphs and adults pierce the leaves and stems with sucking mouthparts in June and July causing circular brown lesions. Affected leaves are often unmarketable. Insecticidal soaps used early when nymphs are small can reduce populations. Populations can also be reduced by controlling susceptible weeds and rotating mint-family herbs to different areas of the farm.

Leafhoppers are an important pest of virtually all herb crops. Many carrot-family herbs are susceptible to aster yellows, a mycoplasma disease transmitted by the aster leafhopper. Affected plants are distorted with multiple branches and are unmarketable. Leafhoppers cause hopper-burn, dieback of leaves from the tips, on several herbs, especially fenugreek (methi). Leafhopper feeding causes a stippling on leaves of most other herbs, which can reduce marketability, but rarely renders them completely unmarketable.

Japanese Beetles are a major pest of basil in certain areas of Ontario. The adult beetles emerge in late July and feed for 1-2 months at the top of the plant, causing ragged holes. They are more of a problem if basil is grown near a perennial grass, on which the immature grubs develop. Japanese beetle traps are available on the market but have not proven effective to control this pest and may actually attract more adults to the area.

Two-spotted spider mites can be a significant problem on many of the mint-family herbs. Mites usually feed on the underside of leaves causing leaves to turn mottled and silvery. Fine webbing is usually present on the underside of the leaves. They are more of a problem in dry years when infested transplants are moved from a greenhouse to the field.

Other insect pests of herbs include aphids and garden fleahoppers (most herbs), leafrollers and spittlebugs (mint family), parsleyworms (carrot family), thrips (chives), and tarnished plant bug (most herbs). These pests do not occur in every year or field, but can build up to damaging levels, especially when large acreages are grown.

DISEASES

Basil downy mildew is a new disease in Ontario, first identified in the field in 2010. It appears as chlorotic (yellow) sections of the leaves defined by the veins, often with grey spores underneath. Once initial symptoms are found in a field the crop can be completely destroyed within a couple of weeks. Control of this disease is only possible through the use of a rotation of the registered pest control products Ranman and Confine. On-going research suggests that the cultivar ‘Medinette’ is slightly less susceptible to the disease than other cultivars. The disease may be delayed by growing basil in an open location with good airflow, with wider plant spacing.

Phomopsis has been identified on oregano/marjoram in Ontario and may also affect sage. Lesions develop on the leaves and stem, causing rapid senescence and collapse of individual stems. Little is known about this disease, and more research is required on its biology and management.

Septoria leaf spot is a significant disease on parsley in Ontario. It appears as round lesions on the leaves with small black specks (pycnidia). It usually affects the crop when extended dew periods occur, often later in the summer. It is especially a concern when the plants are weakened by another factor such as dry conditions, nutritional deficiencies, or root damage. Avoiding these issues will help to reduce the impact of the disease. An unrelated Septoria leaf spot also affects lavender, but has less impact on the crop because leaves of lavender are not harvested.

Bacterial blight often affects cilantro in Ontario. It is characterized by small circular lesions with a darker border. Bacterial diseases are usually spread by rain or irrigation-splashed spores or by machinery or field workers. Sanitation is important with this disease, since there are no products registered for its control.

Other significant diseases of culinary herbs include Phoma blight on dill, rust on mint, anthracnose on basil, powdery mildew on most mint-family herbs, and various Alternaria and Cercospora leaf blights on many mint and carrot-family herbs and fenugreek. Numerous soil-borne diseases also affect herbs, but in most cases have not been identified. Rhizoctonia and Fusarium are the most common fungi associated with crown and root rots. Root knot and root lesion nematodes also affect a wide range of herbs and can build up to significant levels if herbs are not rotated with unsusceptible hosts.

Management of pests of herbs is particularly challenging because few pest control products are registered for use on these crops. Growers should continually scout fields to identify problems before they get out of hand. Some pest issues can be more easily controlled at an early stage when they are isolated to one or a few plants. Sanitation, crop rotation, proper site selection, and fertilization are essential to reduce the chances of a pest developing to damaging levels.

OMAF and MRA staff are working on resources to assist growers with identification of these pests. Information on pests of culinary herbs along with photos of the significant pests are available in the new SPECIALTY CROPportunities module, which is now available on the OMAF Crops website. For more assistance with identification and management of herb pests, growers should contact an OMAF and MRA specialist.