Sunflower pests and diseases
Even though the sunflower can be infested and infected by various pests and diseases, the need for control measures is not very common. However, the threats may vary from region to region and from one year to the next, and the farmer must evaluate the need (economic profit-damage) for any applications. The economic threshold reflects the minimum number-level of insects (or damage) that should be considered a signal for applying control measures to avoid significant yield loss. This number may vary between different insects and crop growth stages. The crop has different critical stages and targeted plant parts that are in danger from specific “enemies.”
The major Sunflower Insects
Chewing insects like grasshoppers, caterpillars of different species or beetles can feed and damage the sunflower leaves. The problem is not so big in most cases, and the farmers do not take action. The efficiency is higher when the farmer adopts integrated management strategies. For many insects, the late sowing is the most effective measure to avoid significant yield losses due to pest infestation. The farmer should stay alert in the case of the sunflower beetle (Zygogramma exclamationis), which in a large population, both adults and larvae can damage cotyledons and first leaves, leading to yield losses. In the seedling stage, the economic threshold is 1-2 adults per plant or 10-15 larvae per plant (1).
Except for the emerging seedlings with minimum leaf surface, the farmer needs to know which are the other “sensitive” growth stages that leaf damage can lead to measurable yield loss. For example, from the R2 to the R6 stage (from 52nd until 92nd day after planting), a 40% leaf destruction will result in more than 15-20% yield loss. More specifically, the R3 growth stage (59-67 days after planting) is the most critical stage concerning leaf reduction since 20-25% defoliation can cause 10% yield loss. The yield loss can increase if the defoliation or stand reduction happens later in the plant’s life. This is because the plants have spent a long time competing with each other for recourses (2).
Up to 6 cutworm species can be found in a sunflower field. Four of them belong to the Euxoa genus, at least one in the Agrotis and Feltia genus. While the insect is feeding on the leaves of sunflower seedlings, the most extended damage is caused by the larvae that can cut off the neck-stem of the young plants, and this usually results in plant death. Mature larvae only infest young plants with tender shoots because, later on, they cannot eat it when the stem is thick and strong. Since pests develop immunity against pesticides easily, biological management is the best method to control them. The control must be performed at crop emergence or before sowing. Once the crop has been attacked, management is more complicated. Chemical management is used only if the problem is severe and always under supervision from a local licensed agronomist. Generally, the economic threshold is 10 larvae per square meter or a 25-30% stand reduction. The farmer needs to scout the field during the night to find the insect (3).
Despite the measurable yield losses that can occur in extreme infestation from the insects mentioned above, the problem is not that common. However, the farmer needs to regularly scout his/her field when the bud development starts. The reason for concern at that stage is the presence and activity of the head clipper insects. More specifically, the sunflower moth is considered the major and most common pest threat for sunflowers (4).
There are 3 major species of sunflower moth that can damage sunflower crops worldwide. The population size and yield losses that they can cause may vary from region to region. They can spread over great distances with the help of the wind, while warm conditions accelerate their life cycle. The management practices are effective when they target the adults and the young larvae before entering and damaging the plant (5).
- Sunflower Moth (Homoeosoma electellum) The larvae feed on the pollen and floret and can lead to failed flower fertilization and empty seeds. In areas with a known history of infestation, the farmers try to avoid major problems by sowing at a later date. For measuring the insect population, the farmer should scout the field, observe and take samples, while sex pheromone lures (traps) can also be used as a side indicator. Chemical control can be applied when we count 1-3 adults per 5 plants at the onset of flowering (R5.1) or within 7 days of the adult moth’s first appearance or 4 moths per trap per day from the R3 through R5 growth stages (1). Each moth larvae can damage up to 10 seeds (6).
- Sunflower Bud Moth (Suleima helianthana) The adults lay eggs on the leaf axils, the flower buds, or the open sunflower heads. The larvae that hatch start feeding in the pith area of either the stalk or head. The insect can usually reach high populations. The yield losses may be significant only when the larvae burrow into unopened buds causing irregular head development. The infestation is more extended (up to 80-85% of stalks) in early plantations.
- Banded Sunflower Moth (Cochylis hospes Walsingham) The adults lay eggs on the outside of the bracts of the unmatured sunflower heads. The sunflower is susceptible only during the flowering stage (R5). Economic damage is caused by the feeding of the larvae of all stages on disk florets and seeds. To calculate the Economic Injury Level by considering the treatment cost and the insect population, the farmer can use specific formulas (read more here 1). In conventional tillage systems, the farmers can implement deep fall plowing. An effective alternative is delayed planting. Finally, natural enemies of the moth (predators and parasitic wasps) can be important allies of the farmer.
Attention: Use only pollinator-friendly insecticides and avoid any spraying during the day when pollinators fly and feed on the blooming sunflowers. Inform any neighbor beekeeper before applying any product. You can discuss this with your local licensed agronomist.
Significant yield losses from birds
Grackles, blackbirds, and other flocking birds can be serious pests in sunflower fields due to their extended feeding with the sunflower seeds. In areas where the bird populations are high, the farmer should avoid planting sunflowers close to wetlands, ponds, cattail (roosting sites for blackbirds ), marshes, woodlots, or tree lines. In most modern sunflower varieties, the heads turn and face the ground after flower fertilization to reduce the yield losses caused by birds. Additionally, early sowing allows early harvesting before the birds congregate in flocks. Generally, the problem can be serious to smaller and more isolated sunflower fields. To scare the birds away from the fields, farmers can use automatic exploders. One device can protect 4-8 hectares (10-20 acres), and it is more efficient when activated before birds begin to arrive from their roosting area at sunrise. The farmer must move it every two or three days to different parts of the field so the birds do not get used to it and lose their efficiency (7).
Major sunflower diseases
More than 30 diseases have been reported to affect sunflower plants. However, only one-fifth of them can cause any significant problems to the yield production of the crop (8). Like insects, the pathogen species and the economic loss they cause may vary depending on the environmental conditions.
Sclerotinia (head) Rot/Wilt (Sclerotinia sclerotiorum), White mold
These fungi are considered the most widespread and damaging pathogen infecting the sunflower. The most common symptom is the development of lesions (soft, mushy, brown) on the back of the head or the other infected plant parts. Gradually the lesions will expand and may be covered by white hyphae. The infection can happen anytime during a sunflower’s growth but is especially damaging during flowering. The fungus growth is favored by cool (18-23oC, 64-73oF) humid weather, and the symptoms can initially be observed in a single plant, a row, or a cluster in the field.
The control of the disease is very hard since the sclerotinia can survive on the soil and crop residues for several years. The farmer should select planting in a sclerotinia-free soil since sunflowers have no available resistant hybrids. Foliar application of fungicides is not recommended due to low efficiency (9). Additionally, over-fertilization should be avoided (10). Crop rotation is a valuable tool, but the farmer should carefully select the species that he/she will include since many crops are possible hosts. More specifically, the pathogen can also infect soybeans, canola, mustard, dry beans, field peas, lentils, and potatoes, which are better to be avoided right before or/and after sunflower (11, 12).
Rust (Puccinia helianthi)
It is one of the most economically important and widespread diseases with the potential to cause significant yield losses (13). The infection and the fungus growth are favored in humid or rainy weather and temperatures above 24 oC (75 oF). Symptoms primarily appear on the upper surface of the lower leaves as small, circular, characteristically orange spots surrounded by a chlorotic halo. Gradually these spots grow and cover almost the entire leaf. Infected plants produce fewer seeds with reduced weight and oil content (6). In general, oil seed-type sunflowers are more susceptible (14).
Disease control begins with proper precautionary measures. These include weed control and early planting, selecting a resistant sunflower hybrid, good crop aeration, proper field drainage, avoiding foliar irrigation, and crop rotation. The general condition of the plants (nutrients and water level, sun exposure) can also boost their immunity. It is also crucial to properly clean and sanitize any equipment used to avoid the fungus spreading in healthy fields or field areas. Chemical treatment is used only if the problem is severe and always under the supervision of a local licensed agronomist. The best time for fungicide application is before or during the early flowering stages of the plants. Some commercially available active substances are cyproconazole, pyraclostrobin, fluxapyroxad, and azoxystrobin (15).
Root and stalk rots caused by bacteria are more common and damaging in wet, heavy soils with a high soil water table. For this reason, it is generally advised to avoid such a field for sunflower cultivation (7).
Sunflower pests and diseases
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