Onion smut

Fig. 1. Infected leaves can become
bloated and split open, releasing spores
and contaminating the surrounding
soil.  Photo taken early August, 2019.

Onion smut only infects Alliums, and is a disease that can lie dormant in the soil for 15+ years, even in the absence of a host crop.  A cold spring such as we had this year results in the onion flag leaf emerging slowly, increasing the likelihood of smut infecting the onion.  The smut fungi can only penetrate and infect young seedlings - if the cotyledon escapes infection until it is mature, no further infection can occur despite high disease pressure. For this reason, healthy transplanted onions or onion sets typically avoid being infected by smut since they do not emerge in infected soils.  The infection period is from two or three days after the seed germinates until the first leaf is about three inches tall (about 10-21 days, depending on the year.)

Fig. 2. Black streaking on the onion bulb, August.

Early infections can result in seedling death, and plants that do survive will have characteristic black spots, blisters, and/or streaking (Fig. 1, 2).  Some infected plants will shed their outer leaves in the second month of growth, and continue growing, free of infection.  However, this year I'm seeing many infected plants that have survived the first few weeks of infection, and the smut fungus has continued to grow, systematically colonizing the plant.  These plants are are usually stunted.  Black pustules can infect as deep as the third or fourth scale (Figure 3).  Many infected plants will dry up, wither, and die by harvest.  Infected bulbs that do make it into storage shrink more rapidly, and are more prone to bacterial rots which can contaminate other bulbs in storage.

The causal fungi, Urocystis magica (synonym: U. cepulae) and U. colchici, can be transmitted from one field to another by surface drainage water, wind-blown soil, tools and farm equipment, soil in your shoe or truck treads, or any other method that transports soil.  Have you read Perennia's fact sheet about biosecurity in horticulture crops?

Fig. 3.  Smut infections penetrating several scales deep.

Long crop rotations are an important tool to limit the build up of disease in the soil.  Conventional growers that are planting into a known infected field should use seed treatments or an in-furrow fungicide application.  If organic or small-scale growers have a field history of onion smut, it is recommended to transplant onions into the field, rather than plant from seed in the infected soil.  Growers with smaller plantings should also remove and destroy diseased seedlings and plants to avoid building disease levels in the soil.

Subscribe to Nova Scotia Vegetable Blog by Email

Hailstorms and crop impact

The hail storm over the weekend caused scattered damage in the Annapolis Valley, as well as in Hants, Colchester, and Cumberland Counties.  The degree of damage depends a lot on location and crop type, as well as on wind velocity, the size and shape of the hailstones, and how long the hail lasted.  The warm, wet weather that followed the storm can exacerbate disease problems, especially bacterial infections.

Fig. 1. Healing hail damage on butternut squash,
two weeks post-storm.

Crops with large leaves such as Cucurbits will typically look pretty destroyed, but, depending on the severity of the damage to the leaves, and how well the fruit were protected, might pull out of it with minimal yield loss.  If the Cucurbits are on a drip system, a little extra nitrogen will encourage the regeneration of leafy growth.  Damaged zucchinis and summer squash should be removed.  Leaving damaged fruits on the plant, which are unlikely to be marketable, will be a drain on the plant's resources and could result in more prolonged yield losses.  Depending on the severity of damage, many winter squash will scar-over (Figure 1, Figure 2).  Healed scars should not pose a disease risk, but may render the fruit undesirable to some buyers.

Fig 2. Healed hail damage on butternut squash,
seven weeks post-storm.

Other crops that are susceptible to bacterial rots (Figure 3) such as onions will have long-lasting damage - crop injury (Figure 4) opens the door for bacterial infection, which quickly spreads down the leaves into the bulbs.  Dead tissue is prime for stemphylium infection.  Growers are encouraged to continue with preventative stemphylium sprays.  Copper may help with bacterial infection, but results have been poor on onions in the past.

Leaf loss in crops close to harvest such as sweet corn or root crops should have minimal effects on yield.  However, direct impact on sweet corn ears can render them unmarketable, depending on the degree of damage.

Fruiting vegetables such as peppers will be more prone to sunburn if protective foliage has been badly damaged.  Fruits that have taken a direct hit, depending on the size of the hail stone and the velocity of the strike, can be unmarketable.

Fig. 3. Bacterial infection spreading down an
onion leaf

Losses of flowers and small fruit will reduce yield and delay harvest in crops such as green beans.

Wounds in plant tissue open the door for diseases to come in.  It is recommended that a protectant such as copper or Bravo be applied (if labeled) for your damaged crop, which will help guard against infection while the plant heals.  Always read and follow the label, the most up to date labels can be found on the Health Canada website.

If you have Crop Insurance, it is recommended that you reach out so that they can document the damage.  Should a claim be necessary down the road, they will have a record of how extensive the damage was, which will make filing a claim easier.  The days just after a hail storm are often the worse.  Apply a protectant, maybe give your crop a bit of extra fertility, if appropriate, and then come back in a week to see how things look.  If the crop pulls out of the damage, then no insurance claim needs to be filed.  Registered farms in Nova Scotia shouldn't hesitate to contact Perennia if you are unsure of best management practices in crops that have been affected by adverse weather.

Fig 4. Hail damage on onions

Subscribe to Nova Scotia Vegetable Blog by Email

Garlic harvest!

Wrapper leaves snug around cloves.

Garlic harvest is upon us, and if you haven't already started harvesting your garlic, now would be the time to start planning it.  Different varieties will mature at different times, so it's always good to check on bulb development of few bulbs of each variety.  One way to determine harvest-readiness is by looking at the number of green leaves.  Each leaf corresponds to a wrapper leaf around the bulbs.  Wrapper leaves protect the cloves from light, moisture, heat, etc.  For best storability, it is suggested to harvest with five to six green leaves.  That way, if a wrapper leaf or three is lost in harvesting and cleaning, there will still be two to three wrapper leaves around the bulb to protect it from storage rots, desiccation, etc.

Stem and bulb nematode damage

Green leaves should be used as a guideline, but not a rule.  Sometimes on a particularly healthy plant, the leaves can remain green despite harvest-readiness.  Alternatively, if there is heavy thrip damage, high stem and bulb nematode populations, etc. the leaves can brown prematurely.  It is always best to pull a few bulbs and check to see how the cloves are filling the wrapper leaves.  Give the bulb a squeeze, and if there is any give, then the garlic isn't quite ready yet.  You should also cut the bulbs in half perpendicularly to the stem.  Each clove should be tight in the wrapper leaves.  If the wrapper leaves seem a little loose around the cloves, then wait a little longer to harvest.  The cloves in harvest-ready garlic will also start to pull slightly away from the stem, especially in hardneck varieties. 

Cloves starting to pull away from stem.

Hardneck varieties (Allium sativum var. ophioscorodon) produce scapes.  Hardneck types, as a general rule of thumb, produce larger cloves, but have a shorter storage life, which can be greatly influenced by harvest timing.  If you leave harvest for too long, the wrapper leaves will start to decay, and the cloves will continue to grow and pull away from the stem and the bulb will split open, rendering the bulb unmarketable.  Softneck varieties (A. sativum var. sativum) do not produce a scape, and typically have a longer storage life.

Garlic harvested too early might not have fully developed its yield potential or flavour profile, and will tend to shrivel when cured.  Late-harvested garlic is more likely to have poor storability, particularly the hardneck varieties, as the wrapper leaves start to deteriorate, exposing cloves.  It is often better to harvest a little early than a little late.

Target harvest for early in the morning on a dry day for best results.  Do not leave garlic in the sun for long as it can scald, and the cloves will quickly deteriorate.  Handle garlic gently as it is sensitive to bruising.  The higher the moisture or relative humidity when you are drying your garlic, the slower the garlic will dry down and cure, resulting in high disease potential.

Post-harvest handling can dramatically affect garlic quality and storability.  Recent research from Cornell University suggests that root trimming does not have any impact on bulb quality, weight, or disease incidence.  Washing garlic post-harvest, while resulting in good looking bulbs initially, ultimately resulted in more discolouration after drying and curing.  For more details about post-harvest handling of garlic, check out Cornell's Garlic Post-Harvest Study.

Subscribe to Nova Scotia Vegetable Blog by Email!