Mid West potatoes: seed production, pest and disease management

Introduction

There is opportunity to expand the fresh, crisp and seed potato industry sectors to meet demand through increased ‘out of season’ or winter production north of Perth in the Mid West of Western Australia between Lancelin and Dongara.

Expansion of winter production will increase the value of the potato industry and increase the period of the year when additional supplies are available. It will also create opportunities for industry to access interstate and export markets counter to the main cropping season in WA.

The focus of production is currently Dandaragan (140km north of Perth and 75km south-east of Jurien Bay) where 160ha were sown by three growers in 2012. Department of Agriculture and Food, Western Australia (DAFWA) data indicates adequate arable land and water of suitable quality to enable increased production.

DAFWA assessed factors limiting yield by monitoring insects and diseases in crops and providing support for agronomic decisions on three commercial properties in 2011 and 2012.

While improvements in agronomic management were identified, the main limitation was seed quality.

This page looks at best management practices for seed production and pest and disease management.

Seed

Pest and disease levels and the physiological age of seed are important in growing successful crops.

In the Mid West, growers are both purchasers and growers of seed. Seed supplied from the South West is too physiologically young for autumn sowings so growers bulk seed in a winter sowing for one generation so it is the right age for planting the following autumn.

The procedure is to plant seed that has been multiplied through four field generations (G4) and harvest the fifth field generation (G5) in spring so it is the right age for planting the following autumn.

For these seed crops it is important to use G4 seed of the highest quality, such as certified seed from the WA scheme, and to manage crops so the resulting G5 is of high quality. High quality source seed and vigilant seed crop management are vital.

Insect monitoring showed aphid numbers to be higher than in traditional summer growing seed areas in the South West. Virus monitoring showed aphid-borne potato leafroll virus (PLRV) was often present where aphid control was excellent. Other viruses such as potato virus S (PVS) were also found in crops grown from seed that was assumed to have very low level of viruses.

Selection

For seed, ware or crisp crops, use high quality seed of the correct variety. For a seed crop bulked once or twice, select higher quality ‘certified’ seed than for a non-seed crop. Choose varieties to suit growing conditions and market requirements.

Growing a non-approved variety could lead to buyer rejection and significant loss of income. Once the right variety is selected, use high quality seed.

Sanitary quality

Seed schemes focus on sanitary quality so that buyers of certified or registered seed can be assured that it meets minimum standards of disease and pest incidence and is of high quality. To minimise the risk of viruses and other infections, use certified or registered seed from Western Australian Seed Potato Producers Inc. (WASPP).

WASPP seed is subject to either the ‘Western Australian Certified Seed Production Rules’ or ‘Western Australian Registered Seed Production Rules’. Application of the rules is overseen by DAFWA inspectors who examine crops twice during growth and tubers once after harvest. Growers can be trained to become Accredited Tuber Inspectors (ATI) which qualifies them to conduct their own inspections.

The number of generations (G) over which seed crops can be multiplied is limited. The maximum is five (G5) for certified seed. Each generation of seed crop must comply with strict rating standards based on field inspections for disease, foreign varieties and permitted disease/nematode infection levels in tubers.

A list of seed growers is published annually in the ‘WA Grower’. This provides information on where a seed crop is grown, generation, variety and expected harvest date. The rating of seed crops can be discussed with seed suppliers.

Updated information may also be obtained from the Senior Seed Certification Officer at the Manjimup Horticultural Research Institute on +61 (0)8 9777 0000.

Physiological age

In the Mid West, the main crop is autumn-planted for which seed of an appropriate physiological age is not readily available. In WA, most seed potatoes are produced in the South West and harvested in summer/autumn to suit sowing from winter to spring. This seed is unsuitable for early autumn sowing. Growers must either obtain seed from producers who can supply seed at the correct age or produce their own.

Own-seed growers will need to grow their crops for at least one generation, usually from G4 to G5, sowing in winter, harvesting in spring and storing over summer. This seed will be an appropriate physiological age for rapid emergence the following autumn.

If  you plan to sell a portion of a commercial crop for seed (for example, small fraction of Atlantic crisp crop for seed export) you must buy in G3 seed which, after the autumn multiplication, will produce G4 seed which can be used to produce G5 (oldest generation permitted for certified seed) from your commercial crop.

Although physiological age is important, it is not assessed by seed schemes. It is related to dormancy and therefore planting time, emergence and market requirements for tuber size and number. It can be inferred from harvest date, postharvest storage conditions and, after sprouting, from sprout number, length and the position on the tuber.

There is also opportunity to assess germination percentage and adjust seeding to account for low rates. Low germination percentage may indicate seed that is too young (dormant) or too old (senile).

Selecting quality seed

It is important that G3 or G4 seed purchased for bulking in the Mid West be of high quality because of greater aphid infestation and possible virus transmittance compared with traditional seed areas. Within the WA seed potato schemes, growers can choose seed with different disease ratings determined during field inspection.

At each inspection, the crop is given a rating of 1 to 3, with rating 1 having the lowest disease incidence and rating 3 having the highest allowable. As Mid West growers will need to grow their own seed for at least one generation, buy seed of rating 1 (see Table 1) to be sure of achieving high quality seed, after one or two bulkings — usually from G3 or G4 to G5 — for subsequent commercial crops.

* All rating 1 crops must be 0% for potato virus Y at the first and second crop inspections.

Rating 1 seed has the lowest tolerance level for virus infection (10 and 100 times less than rating 2 or 3 seed respectively) and therefore represents the greatest chance that the virus will not be introduced to farms where seed is produced.

Planting

Planting time will vary according to growing conditions and the planned harvest or delivery date, which is set by market requirements.

In sandy soils, planting before early March is risky due to possible poor germination in hot soil. Similarly, harvesting in hot conditions after mid-November may result in excess harvest damage.

Seed type

Whole seed is preferred for sowing in warmer autumn months as cut seed has higher risk of breakdown in hot soil. Cut seed is an option for winter sowings. In- and between-row spacings will be determined by variety and market requirements and it is important to get them as accurate as possible.

Planting at too low or too high a density may lower yield, leading to economic loss. Check the spacing between seeds and rows early in the planting operation and adjust as necessary. Be sure you know the likely germination per cent or dormancy of the seed so you can adjust seed rate for low emergence.

Treatment

Seed is usually treated with fungicide to protect it from diseases such as Rhizoctonia and Fusarium in the early stages.

In warm sandy soils, use dry powder rather than liquid formulations to minimise the risk of seed breakdown that could result from  excess water on the tuber surface.

Insect pest management

To produce quality seed and consumer potato crops, follow the principles of integrated pest management:

• Gain confidence to recognise pests, beneficials and benign insects.
• Monitor weekly.
• Base decisions on insecticide use on action thresholds for pest abundance that vary for seed and consumer potato crops.
• Be aware of registered insecticides and their toxicity to beneficials.

If at the time of inspection, any aphids (green peach aphid, Myzus persicae or potato aphid, Macrosiphum euphorbiae) are found in the crop, the grower will be advised that aphid control has not been sufficiently effective and that steps must be taken.

If aphid numbers are considered excessive, the following action will be taken:

• A sample of 100 randomly selected leaves (a middle and lower leaf from 50 plants) will be examined for the presence of aphid colonies. A colony is defined as a leaf containing three or more aphids of which at least two are wingless.
• If more than five colonies are found, the crop will either be rejected immediately or the grower may choose to submit a sample to test for potato leafroll virus (PLRV) and potato virus y (PVY).

If an excessive number of aphids is found at the first inspection, leaves may be sampled at the second inspection and tested for aphid-transmitted viruses. If excess aphids are found at the second inspection, tubers will be sampled at harvest and tested. The number of samples taken must be adequate to determine compliance with the tolerances set out in the WA Certified Seed Potato Production Scheme Rules, July 2013.

Make yourself a monitoring kit which includes the identification guide references listed in Emery et al. 2005 (see Table 3 for a list of pests and page numbers) and the video Managing Virus Diseases of Potato Crops: Potato leafroll and tomato spotted wilt (2003). Potato tuber moth and leaf eating ladybird are minor pests in the Mid West. Photos of these insects do not appear in these guides.

A ‘bum bag’ is also useful containing a 10x hand lens, collecting vials, notebook (waterproof paper and 2B pencil, paper bags and self-seal plastic bags. Monitoring information should be recorded electronically to allow comparisons across seasons.

To protect seed potato crops planted in autumn (March to early April) from early aphid invasion, apply systemic insecticide to the soil at planting. This does not appear to be necessary for winter-planted crops because aphids usually appear later in the crop cycle by which time the activity of any seed applied insecticide would be too low to be effective. Soil-applied insecticide at planting is not required for ware crops because virus levels are less important compared with seed potato crops and natural enemies, especially wasps, help reduce aphid numbers to acceptable levels.

For both ware and seed potato crops, follow a regular insect monitoring program from crop emergence to die-off.

In centre pivots, check two opposite quadrants of the planted area. Follow a circular path through the crop quadrant to ensure a representative area is covered. Monitoring should include 50 destructively sampled leaves in each quadrant up to canopy row closure then 25 leaves thereafter.

Check the undersides of lower, healthy leaves for aphids (Figure 1) and aphid mummies (parasitised aphids) and thrips. Record the occurrence of other insects on leaves and the presence of pests such as potato moth, tobacco looper, Rutherglen bug, Heliothis and beneficial insects such as ladybirds, lacewings, predatory bugs and beetles, and hover fly larvae.

Figure 1 Monitor the underside of leaves for insects such as green peach aphid with nymph present here

For seed crops to the start of senescence, apply aphicides if aphids are present at up to 5% of leaves infested with winged and wingless aphids.

Be aware of the different toxicity levels to beneficial insects of registered insecticides and choose the least toxic (see Table 4). To avoid developing resistance, alternate insecticides. There have been reports of green peach aphid developing resistance to pirimicarb.

Roguing (seed crops only)

Before seed crops reach row closure, undesirable plants should be removed by roguing. Tips on how to rogue crops can be seen on a video (Managing Virus Diseases of Potato Crops: Potato leafroll and tomato spotted wilt 2003).

Disease management

Skin blemish diseases at harvest or postharvest downgrade the packouts of ware potatoes, and some reduce yield. Black dot (Figure 2) and silver scurf (Figure 3) can be particularly prominent in some years, while scab diseases occasionally occur in sandy soils (Figure 4). Control generally begins at planting.

Figure 2 Black dot skin blemish lesion which gets its name from the small black sclerotes (dots) in the lesion

Figure 3 Silver scurf on Royal blue potatoes has a silvery shine when viewed with lights and does not contain the small black sclerotes present in the centre of the lesion

Figure 4 Common scab is caused by a bacterium that produces roughly circular lesions on the tuber surface that are slightly sunken at the centre with flakes of skins surrounding the lesion

Some in-furrow or seed piece chemical applications are registered for the control or suppression of these diseases.

Make sure that tubers are at the correct temperature at planting to prevent sweating or excess surface moisture. Applying liquid fungicides to tubers that are sweating could promote seed piece breakdown. Some of these chemicals are registered for in-crop use so consider resistance management when deciding which chemical to use.

Depending on weather conditions, the crop may be susceptible to foliar diseases that could reduce overall yield. Frosts or other mechanical damage can cause entry points for diseases such as botrytis or early blight (target spot).

Figure 5 Early blight (target spot) of potatoes caused by Alternaria species. Note the concentric growth rings in the lesions.

Overwatering can lead to black leg and Sclerotinia (white mould) diseases as it increases humidity and stresses the plant, reducing its ability to naturally defend against infection.

Over-application of nitrogen fertiliser can lead to excess plant growth. Weak plant cells make crops more susceptible to disease.

Early blight is the most common foliar disease in WA  and the first signs are small circular spots that expand in concentric rings (Figure 5). Older leaves are usually infected first but the disease, if left unmanaged, can lead to severe infection that reduces the crop’s lifespan and subsequent yield.

Numerous chemicals are registered for control of foliar diseases, with several modes of action. Some work as contact fungicides, providing a barrier from infection. Others have translaminar activity where they penetrate the leaf and travel a small distance beyond the point of spray. Foliar diseases of potatoes can develop resistance to the chemicals used to control them so follow the resistance management guidelines for potatoes to prevent this from occurring. The guidelines are published by Croplife Australia.  Always read the label of the fungicide thoroughly prior to application.

Acknowledgement

Much information in this publication is based on results from a number of potato research and development  projects funded by Horticulture Australia Limited with voluntary contributions from APC Potato Producers Committee, matched by the Australian Government and with in-kind contributions from and managed by the Department of Agriculture and Food.