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Key Weeds to Keep an Eye Out For

Potato crops are susceptible to a variety of weeds and diseases that can impact on crop yield and quality.

The earliest intervention should be seen as a priority, before weeds or diseases have an opportunity to overtake the crop. Such a proactive approach allows greater opportunity for growers to sustain high crop yields and maximise the return on investment.

The benefits of effective weed control are many, including better use of available nutrients by the crop, resulting in increased yields and profits.

The underlying principle of weed control in potato crops is that weed competition can result in a reduction in marketable yield and quality.

• For each 10 per cent increase in weed biomass from annual weeds, potatoes can incur a 12 per cent decrease in yield (Nelson and Thoreson 1981)
• The critical period for weed removal in potatoes is around four to six weeks after planting.

In the Australian market, there are very few herbicide options for selective broadleaf and grass weed control that are suitable for use during early crop development and prior to row closure.

Today, greater importance is placed on limiting the risk of herbicide resistance for potato growers. Herbicides with new active ingredients are essential for resistance management, to broaden the spectrum of weeds controlled and to deliver choice and adaptability.

Over-reliance on any one particular herbicide group will not only select for resistance, but may also change the weed spectrum, thereby allowing a weed or weed species not controlled by that particular herbicide group, to become dominant.
This means the adoption and implementation of effective, long-term weed control strategies that have an integrated and planned approach and do not rely solely on the use of herbicides to control weeds.

An integrated weed management program includes:

• Controlling weeds in the rotational crops or during fallow periods using herbicides with different modes of action;
• Using knockdown herbicides before establishing the crop;
• Where possible, using mechanical weed control before row closure;
• Sowing vigorous crops that compete strongly and quickly close the row over to reduce the amount of light reaching germinating weeds;
• Maintaining a healthy crop stand that can out compete weeds;
• Applying knockdown herbicides as the crop is in the senescent stage to control weed growth and for ease of harvest;
• Rotating potato crops with pasture or other crops that allow easy control of major potato weeds;
• Avoiding use of contaminated seed or unclean machinery. These are often the cause of introduced weed seed; and
• Pre-planting management can greatly help to reduce the weed seed bank. Before planting, form seedbeds and pre-irrigate to encourage weed seeds to germinate, resulting in an easier weed kill.

KEY WEEDS

Annual Ryegrass (Lolium rigidum)

Factors that make Annual Ryegrass a major weed:

• Annual Ryegrass produces an extremely high number of seeds per plant
• Survivors of control measures can tiller well and produce high numbers of viable seed
• These factors result in large seed banks and, subsequently, high weed numbers at emergence. Dense stands (greater than 100 plants/m2) can produce up to 45,000 seeds/m2 under ideal conditions
• Newly formed Annual Ryegrass seeds are typically dormant, with seeds losing dormancy during the first six months after dispersal
• Many populations of Annual Ryegrass have developed resistance to both selective and non-selective herbicides
• In 2013 in Australia, Annual Ryegrass had developed resistance to seven herbicide mode-of-action (MOA) Groups (A, B, C, D, L, M and Q)
• Repeated use of herbicides from the same MOA group (particularly the high-risk Groups A and B) is likely to result in herbicide resistance causing the production of large numbers of seeds and can quickly become a serious and significant weed problem.

Ideal germination Conditions

• Autumn and winter rain events of at least 20 mm and seed located at a depth of 20 mm in the soil. Germination reduces with increasing depth of seed, ceasing at about 100 mm
• The optimum temperature for germination of Annual Ryegrass is much lower for buried seeds in darkness (11°C) compared with seeds in the light (27°C)
• 75 to 80 per cent of the seed will germinate on the first or second significant autumn rain
• 12 to 23 per cent of seed germinates after June (McGowan, 1970). Spike initiation occurs after a cold requirement has been satisfied and the day length increases to more than 8 hours

Biological control

A fungal pathogen (Pyrenophora seminiperda) has been isolated and identified as a potential bioherbicide for grass weeds in the future. The fungus infects the seed, reducing germination, emergence and seedling vigour.

Physical control

Cultivation and mowing can be implemented for several consecutive years prior to planting to reduce the seed bank.

Conventional control

Herbicide resistance developed rapidly in Annual Ryegrass because of high genetic variability, high reproduction by outcrossing (cross pollination by wind allows gene transfer from resistant to susceptible plants via pollen).

There are now Annual Ryegrass populations resistant to almost all herbicides available for its control.

Silverleaf Nightshade (Solanum elaeagnifolium)

Factors that make Silverleaf Nightshade a major weed:

• Silverleaf Nightshade belongs to the genus Solanum which includes potatoes and tomatoes
• Silverleaf Nightshade is a declared noxious weed in New South Wales, Victoria and South Australia
• Silverleaf Nightshade can reproduce from roots or seeds. Seeds are produced in four to eight weeks after flowering
• Cultivation cuts the roots into pieces and aids the transport of material that could form new infestations
• Seed bank estimates range from 1,200 to 25,000 m2, although Australian records indicate 4,000 m2 is more likely
• Seeds have a high viability and persist for several years, but generally require warmth and rainfall to germinate
• Seeds are dispersed primarily by livestock, but can also be spread by water, birds, vehicles and machinery.

Ideal germination conditions

Silverleaf Nightshade prefers temperate regions with annual rainfall between 250 to 600 mm. It will grow in most soil types and favours disturbed areas such as cultivated paddocks and roadsides. It is sensitive to frost, which will kill new shoots, but is highly drought resistant due to its extensive root system.

Biological control

Due to Silverleaf Nightshade being closely related to tomatoes, eggplant, capsicum and tobacco, biological agents are not considered an option.

Physical control

Ensure any machinery, hay and livestock brought into cropping paddocks are weed-free. Also ensure control measures are implemented on areas around your planned cropping area e.g. roadsides, surrounding paddocks etc.

Conventional control

Silverleaf Nightshade infestations cannot be successfully eradicated with a single herbicide application. Foliar growth can be controlled in season, however regrowth can occur later in the same season or in the next season from the same rootbank.

Maintaining a herbicide program over several years is required to run down an established Silverleaf Nightshade problem due to the extreme persistence of seeds and roots in soil. Broadleaf, contact and pre-emergent herbicides are effective if applied carefully before seed is set.

Blackberry Nightshade (Solanum nigrum)

Factors that make Blackberry Nightshade a major weed:

• Blackberry Nightshade belongs to the genus Solanum which includes potatoes and tomatoes
• Blackberry Nightshade (Solanum nigrum) is regarded as an environmental weed in Victoria, Western Australia, New South Wales, Queensland and the Northern Territory
• Seed bank estimates range from 1,200 to 25,000 m2, although Australian records indicate 4,000 m2 is more likely
• Seeds have a high viability and persist for several years, but generally require warmth and rainfall to germinate
• Seeds are dispersed primarily by livestock, but can also be spread by water, birds, vehicles and machinery

Ideal germination conditions

Blackberry Nightshade is considered to be an annual or short-lived perennial. It emerges in autumn and winter and can last through summer to the following season and live as a perennial plant.

Biological control

There is no known biological control. Grazing is not recommended as fruit can be toxic to livestock.

Fat Hen (Chenopodium album)

Factors that make Fat Hen a major weed:

• Fat Hen grows faster and absorbs nutrients more efficiently than any crop and can grow in almost any soil
• Plants have produced 500,000 seeds and seeds have been known to survive for 30 to 40 years in the soil

Ideal germination and establishment conditions

Fat Hen is a plant that grows well under a wide range of environmental conditions (it does, however, dislike shady environments). Fat Hen prefers moderately fertile soil and tolerates pH in the range 4.5 to 8.3. It can grow year round. In the northern areas, it tends to be a winter weed, but it is more a summer weed in the southern areas.

Biological control

Fat Hen is vulnerable to leaf miners (small insects that eat its leaves). It does not compete well with other groundcovers and suppressing it with cover crops can help.

Physical control

Manual removal, particularly when plants are young, is effective because Fat Hen generally produces a shallow root system. Mulching is an excellent method to prevent germinating seeds from establishing. Cultivation may be useful, again because the plant’s shallow root system is easily dislodged from the soil and the plant quickly dries out in warm weather.

Conventional control

Broadleaf, contact and pre-emergent herbicides are effective when applied carefully before seed is set.

Couch Grass (Cynodon dactylon)

Factors that make Couch Grass a major weed:

• The lack of regular cultivation and its ability to spread its tough scaly rhizomes and branching stolons, make Couch Grass a very difficult weed to control
• Because of its rapid growth and aggressive growth habit, Couch Grass is categorised as a noxious weed in some states
• Couch Grass will effectively smother low vigour crops
• Apical dominance makes it hard to control with herbicides

Ideal germination and establishment conditions

Couch Grass can grow all year round if water is available, but in general, it is a summer weed that thrives on irrigation. It spreads rapidly by seed and runners and even small fragments of roots.

Biological control

There is no effective biological control of Couch Grass. It can be grazed heavily but will continue to survive into the following season, spreading from the root system.

Physical control

Hand removal must be complete or the plants can spread from root sections left over in the soil. Cultivation can spread this grass and is not recommended.

Conventional control

Grass selective herbicides are effective on Couch Grass. Best results come from a ‘Double Knock’ spray program of SPRAY.SEED (non-selective) followed by FUSILADE FORTE (selective herbicide).