Potato Diseases

What is late blight in potatoes?

Late blight is a polycyclic fungal disease of potatoes that thrives in wet, humid conditions in the summer and can rapidly spread, causing severe yield losses.

What are the two forms of late blight in potatoes?

Foliar blight (indirect yield loss) and tuber blight (marketing and storage problems)

Why do potato growers spend so much money on protectant fungicides for late blight?

Fungicides are necessary to prevent infections destroying large portions of the crop (cost effective)

Where does late blight often first appear in a field?

In sheltered areas near trees, where humidity remains high and creates favourable conditions for the disease to develop.

What visible symptoms on leaves indicate late blight infection?

Dark necrotic patches on leaves, along with fluffy fungal sporulation on the undersides which blows on the wind to infect further foliage

When does late blight thrive most strongly?

This disease flourishes in wet summers, rapidly developing under humid conditions and spreading to adjacent foliage.

How can infected tubers contribute to late blight outbreaks the following season?

Tubers carrying the pathogen produce stem blight as they grow, releasing spores that blow into the crop causing potato blight

Which two types of life cycle does late blight have?

Late blight reproduces asexually (most common) and can also reproduce sexually if different mating types come into contact on infected tissue.

How does late blight overwinter?

It survives in infected tubers found in cull piles, on volunteer plants, or those used as seed, remaining dormant until conditions allow new sporulation.

What happens when diseased tubers infected with late blight gives rise to infected plants and the fungus starts producing sporangiophores in warm and humid conditions?

The sporangium land on further leaves within the crop and germinate directly on the leaf surface to create new spores that reinfect other leaves/plants every 7 days.

What will form in cold and wet conditions when looking at late blight sporangiophores?

They form zoospores, which may infect leaves or wash into the soil and infect tubers

Why is zoospore formation significant for potato tuber infection?

Zoospores move with water and can infect tubers in the soil, surviving in cull piles, as volunteers or those used as seed and therefore the late blight lifecycle is repeated

How does late blight's sexual cycle begin in the field?

Two sporangia of different mating types (A1 and A2) fuse on an infected leaf, producing oospores that can survive in soil for many years.

Why can oospores formed in late blight’s sexual cycle be problematic for future potato crops?

They persist in the soil over long periods, eventually germinate, releasing sporangium and zoospores which infect new plants

How can genetic variation from sexual reproduction affect late blight control?

New strains could show resistance to fungicides and reduce the effectiveness of resistant varieties

What are the key IPM steps to prevent late blight infection from cull piles?

Keep cull piles away from fields and cover them with thick polythene so they cannot sprout, preventing the fungus from sporulating and spreading into the crop

Why should volunteers and crop debris be removed before planting the next potato crop?

Volunteer plants, even if small, can harbour the pathogen and produce sporangiophores that move onto newly planted crops.

How does good seed hygiene help with late blight management?

Using certified or tested seed reduces the risk of planting tubers already harbouring the late blight pathogen.

Why is machinery hygiene important in controlling late blight?

Contaminated equipment can carry infected debris or soil to otherwise clean fields, introducing the disease in new locations.

How can the choice of potato variety influence late blight risk?

Resistant or tolerant varieties can to both foliar and tuber blight prevent/delay disease onset, reduce its spread, and minimise yield losses

What is an example of a potato variety with moderate resistance to foliar blight?

Desiree (4)

How can crop health help prevent damage from late blight in potatoes?

Ensuring the crop is as healthy as possible through good nutrition, adequate seedbed quality, drilling date etc can reduce impact from late blight

Why should environmental conditions be monitored for late blight risk?

Higher humidity, leaf wetness, and warm temperatures signal ideal conditions for late blight, guiding cultural interventions and timely fungicide sprays

What is the Hutton criteria which indicates when late blight is more likely to occur?

For two consecutive days:

• Min temp >= 10°C

• Min of 6 hours each day with RH >= 90%

Which online programme can be used to pinpoint locations where conditions are optimum for late blight to occur?

BlightSpray

What role does irrigation management play in preventing late blight?

Minimising leaks and avoiding excessive wetness lowers humidity in the crop canopy, slowing disease development.

Why is timely haulm destruction useful for late blight control?

Removing above-ground growth once tubers have reached marketable size prevents continued sporulation on dying foliage.

How does field location or microclimate affect late blight risk?

Fields surrounded by trees or hedges may have higher humidity, promoting faster late blight establishment and spread.

How should potato canopy density be managed to reduce the risk of late blight infection?

Denser canopies will have greater humidity and so seed rate and furrow separation should be managed appropriately

Why should growers only plant as many potatoes as they can effectively spray when looking at late blight control?

Areas left unsprayed become infection hot spots, jeopardising adjacent plants and potentially affecting the entire field.

How do fungicides fit into late blight management?

They are used on a preventative schedule, requiring overlapping applications to keep leaves protected at all times.

Why must fungicide sprays for late blight in potatoes overlap in timing up until haulm desiccation?

Any gap in protective coverage leaves foliage open to infection by the rapidly spreading pathogen.

What must you ensure in terms of products used and rates applied when applying fungicides for late blight control?

Use the manufacturers full recommended rate and do not mix multiple products together at lower rates as this will contribute to resistance

What is essential for effective fungicide coverage against late blight?

• Sufficient water volume

• Fine spray quality to ensure droplets can cover the underside of the leaf

Why should multiple modes of action be used in fungicide programs?

Using different chemistries helps to protect site specific molecules and reduces the risk of resistance

Which three late blight strains have demonstrated fungicide resistance?

• EU-13-A2 (Blue 13)

• EU-37-A2

• EU-32-A1

Which type of late blight does the EU-13-A2 genotype correlate to?

Foliar blight

Which active/product is the foliar blight strain EU-13-A2 (Blue 13) completely resistant towards?

Phenylamides such as Metalaxyl-M found in the product Fubol Gold WG (with mancozeb)

Which active/product does the late blight strain EU-37-A2 show reduced sensitivity towards?

Fluazinam found in the product Shrilan

Which active/product is the late blight strain EU-32-A1 completely resistant towards?

Mandipropamid found in the product Carial Flex - belong to the “Carboxylic Acid Amide” MOA group

What do 'contact', 'translaminar,' and 'systemic' mean in fungicides?

• Contact = All actives on the outside of the leaf and take out any germinating blight spore

• Translaminar = At least one active will move into the leaf (acts locally and not from leaf to leaf)

• Systemic = At least one active that can move up the plant

What is an example of a contact fungicide used for late blight control?

Ranman Top (cyazofamid)

What is an example of a translaminar fungicide used for late blight control?

Revus (mandipropamid)

What is an example of a systemic fungicide used for late blight control?

Zorvec Enicade (oxathiapiprolin)

What is the 1st phase in potato growth when looking at a late blight fungicide spray programme?

Crop emergence to rapid haulm growth

What is the 2nd phase in potato growth when looking at a late blight fungicide spray programme?

Rapid haulm growth (new leaf emerging every 5 days)

What is the 3rd phase in potato growth when looking at a late blight fungicide spray programme?

Stable canopy phase

What is the 4th phase in potato growth when looking at a late blight fungicide spray programme?

Senescence and haulm desiccation

When are contact fungicides typically used in a late blight spray program?

They are commonly applied from crop emergence until the onset of rapid haulm growth (1st phase) when leaf expansion is relatively slow.

What is an example of a contact fungicide that can be used in the 1st phase of a late blight spray programme?

Ranman Top (cyazofamid)

Why do growers switch to systemic products during rapid haulm growth (2nd phase) in a late blight spray programme?

Systemic actives protect new leaves that appear too quickly for contact-only solutions to shield in time.

What was the old standard of fungicides used in the 2nd phase of a late blight spray programme which led to resistance buildup?

Shirlan (fluazinam) used to be sprayed every 7 days (contact activity only) - risky as weather may change and prevent application - also contributed to resistance buildup

What is an example of a systemic fungicide that can be used in the 2nd phase of a late blight spray programme?

Zorvec Enicade (oxathiapiprolin)

What is the stable canopy phase (3rd phase) in late blight management?

It is when haulm growth slows and growers often rely on contact or translaminar sprays rather than systemic options to manage cost and resistance issues.

Why would a translaminar product be used in the 3rd phase of a late blight spray programme?

If conditions are favourable for blight and pressures are high (contact sprays wont offer enough protection)

What is an example of a translaminar fungicide that can be used in the 3rd phase of a late blight spray programme?

Revus (mandipropamid)

What is “kick back” in relation to late blight fungicide control?

Where translaminar and systemic actives move in the leaf and kill late blight when in it’s latent phase (48 hours from infection until symptoms develop)

What is a benefit of using translaminar fungicides over contact in a late blight spray programme?

They improve coverage as they will move to the underside of the leaf - also less prone to weathering after the rainfastness period as the product will be in the leaf and not on it

Why do late blight sprays continue through haulm destruction and senescence (4th phase)?

Crop remains vulnerable to foliar blight during senescence and any unprotected leaves or stems can still develop blight that may wash spores into the soil and infect tubers - spray should protect the leaf and also target zoospores in the soil.

What are three examples of actives that have shown to reduce tuber blight?

Fluazinam, Zoxamide and Cyazofamid

How long should potato harvesting begin after haulm death?

Minimum 14 days afterwards

What are the symptoms of early blight (alternaria) in potatoes and what cconditions does this prefer?

Brown flecks which enlarge into lesions with concentric ring patterns on the leaves, causing a reduction in leaf area, often linked to warm humid weather and stressed plants (lack of N or manganese deficiency)

How can early blight cause yield loss under severe conditions?

Severe defoliation can reduce photosynthetic area, leading to yield reductions of up to 30% (not as damaging/common as late blight)

Where does early blight survive and how does it spread?

The fungus persists in the soil, volunteers, crop debris, and in infected tubers, with its spores spreading through the crop by wind blown spores and rainsplash.

How does early blight survive overwinter?

In the soil, volunteers, crop debris, and in infected tubers

How do early blight spores produced on crop debris infect the potato crop?

By wind blown spores and via rainsplash

What do early blight spores require to infect new leaves?

Free water on the leaves

What does it mean for early blight to be polycyclic?

It can go through multiple infection cycles in one season, releasing ascospores which infect further leaves within the crop (every 7 to 10 days under favourable conditions - alternate wet and dry periods between 5 and 30 degrees)

What conditions is sporulation favoured by early blights polycyclic lifecycle?

Alternate wet and dry periods between 5 and 30 degrees - lifecycle of 7 to 10 days if conditions are optimum

What happens if tubers become infected with early blight during harvest/lifting?

They rarely develop symptoms until stored where they will show circular sunken lesions

Why does plant stress, such as nutrient deficiency, increase early blight risk?

Stressed crops have lower disease resistance and are more susceptible to infection and faster pathogen growth.

Which non-chemical controls are available for early blight?

Wider rotation, good crop nutrition and optimal irrigation (don’t make the leaves too wet)

Which fungicides can target both early and late blight?

• Vendetta (azoxystrobin and fluazinam) - targets early and late blight, however resistance shown in late blight to fluazinam

• Tank mix Amistar (azoxystrobin) with another late blight fungicide (eg: Ranman Top, Revus or Zorvec Enicade)

What is black scurf and stem canker caused by?

They result from rhizoctonia solani, a seed and soil borne fungus that produces black micro-sclerotia on tubers that overwinters and kills shoots as they emerge - those that do emerge develop stem canker

Why can black scurf be economically damaging?

The black sclerotia cause cosmetic damage, kills shoots as they emerge and causes stem canker in those that do, lowering overall yields.

How does black scurf spread to daughter tubers?

The fungus spreads down the stems and along the stolons, infecting neighbouring tubers to form micro-sclerotia on their surface (black scurfs) leading to cosmetic damage and impacting yield due to stem canker.

Where does black scurf overwinter?

As micro-sclerotia on tubers, crop debris and in the soil

What happens to tubers infected with black scurf?

Emerging shoots will be killed by fungal attack, leading to gaps in the crop, and those that do emerge will develop stem canker

What are three examples of cultural controls that can help suppress damage caused from black scurf?

• Length of rotation

• Seed hygiene

• Optimum nutrition for vigorous crop growth to reduce incidence of stem canker

Which two seed treatments can help manage black scurf?

• Maxim 100 FS (fludioxinil) - not in seed crops

• Rhino DS (flutolanil)

Which two fungicides can be applied in-furrow at planting to control black scurf?

• Amistar (azoxystrobin)

• Allstar (fluapuroxad) - SDHI

What are black leg and soft rot in potatoes?

Pectobacterium bacteria cause dark, wet rotting stems of young plants causing death of plant pre or post-em (black leg) and decaying tubers when stored (soft rot) - leads to gaps in the field with no crop

What causes black leg and soft rot in potatoes?

Pectobacterium bacteria which overwinter in the breathing pores (lenticels) of the potato seed tuber

How do cold and wet conditions when planting favour black leg and soft rot development?

It leads to slow crop emergence which provides more time for bacteria to multiply in seed tubers, spreading into shoots and up the stem.

How do black leg symptoms develop on the infected potato tuber?

The bacteria spread from the tuber to the shoots, exposing the bacteria on the base of the stem which is spread to further crops by rain splash and through insect vectors (also causes gaps in the crop where plants have died from infection)

How can black leg and soft rot spread to the stems of healthy plants and tubers?

By rain splash and through insect vectors from the exposed bacteria on the base of the stem of the initial infected tuber - leads to further black leg in the stem and the bacteria then work down the stolons to infect daughter tubers causing soft rot when stored

What are key measures to control black leg and soft rot?

• Use seed with low bacterial loading

• Store tubers in dry and clean conditions

• Grow low susceptibility varieties (Kestrel = 8)

• Avoid planting in cold and wet conditions

• Harvest early in good soil conditions to reduce soft rot risk

What is common scab and why is it problematic for ware potatoes?

A bacterial disease that forms raised angular scab lesions on tubers, leading to rejections in fresh markets due to poor appearance.

Will the level of common scan on seed tubers effect that on the daughter tubers?

No

Which soil conditions worsen common scab problems?

High pH, sandy soils, or liming before the potato crop is planted (should apply lime at a different point in the rotation)

Which cultural controls can reduce damage caused by common scab in potatoes?

• Grow resistant varieties (King Edwards = 7)

• Avoid liming prior to planting

• Keep soil moist during tuber initiation

What is an example of two potato varieties which have high and low susceptibility to common scab?

• High susceptibility - Maris Piper (1)

• Low susceptibility - King Edwards (7)

Are there any authorised PPP for common scab control?

No

What is powdery scab in potatoes?

It is caused by a fungus that produces black powdery spores on tubers, leading to skin blemishes and tumour-like distortions, forming spore balls that can persist in tubers and in the soil.

Which virus is powdery scab a vector of in potatoes?

Potato Mop Top Virus (PMTV) which causes spraing

How does powdery scab overwinter and infect tubers?

Resting spores overwinter in tubers and in the soil which germinate to produce primary zoospores which infect via root hairs, lenticels and wounds causing lesions on developing tubers. Secondary zoospores are then produced which cause further lesions to form.

When does the tumorous phase of powdery scab occur and what does this cause?

Tumours will develop in wet soils which leads to the formation of spore balls of resting spores on seed tubers and in decaying tissue.

Which cultural control strategies help limit powdery scab?

Growers should avoid excessive irrigation, select resistant varieties like Sante (8), and carefully manage soil moisture during tuber development (requires adequate drainage and avoid compaction).

What fungicides are available for powdery scab control in potatoes?

No fungicides are available in ware crops, however Shirlan (fluazilam) can be applied to the soil at planting under EAMU in seed crops

What is silver scurf and when does it develop?

It is a fungal disease present in the field that causes skin blemishes on tubers (economic impact) when stored if humidity is high.