Soil and plant health in Ohio produce growing

Blueberry fruit rot rarely announces itself in the field. You pick what looks like a clean crop, pack it, and two days later the flats are leaking orange spores and sunken berries. The fungi behind Alternaria, anthracnose, and Botrytis fruit rots work on different clocks, and knowing which one is in your planting changes what you prune, what you plant, and how fast the fruit has to move after harvest.

The details cited in this entry were checked on these three blueberry fruit rots.

Three diseases do most of the damage, and each one has its own timing. Alternaria fruit rot is the most common and severe postharvest rot of blueberry, though it also shows up in the field on overripe fruit. Anthracnose, also called ripe rot, is a serious disease both before and after harvest. Botrytis fruit rot is usually minor but can turn severe in the right weather. All three end the same way, in fruit no buyer will take.

What each rot looks like on the berry

Alternaria comes in first as sunken lesions on the berries. Later a gray-green mass of fungal mycelium and dark green spores appears on the surface. The same fungus can mark leaves with small round necrotic spots about a quarter inch across, edged in reddish brown, and heavy leaf infection can strip a bush.

Anthracnose moves faster. Berries decay rapidly and ooze an orange mass of spores. Hold fruit at room temperature and symptoms can develop in as little as two days. Dark brown lesions can form on twigs and end up ringed by reproductive structures, and reddish brown lesions can appear on infected leaves.

Botrytis shows gray-tan mycelium, fluffy in appearance, with spore masses raised on branched stalks. That mycelium is grayer and less dense than the greener Alternaria growth, which is the quickest way to tell the two apart on a packed flat. Botrytis can also blight leaves, blossoms, and twigs. If no spores are present on symptomatic shoots, the disease is easily confused with the shoot blight stage of mummy berry or with bacterial blight.

Not every blemish is a fruit rot. Damage from excessive handling, from handling wet fruit, or from heat, cold, or chemical injury can look like rot, and a wet harvest can bruise berries in ways that mimic disease. The separation matters, because rot calls for a spray or a cultivar decision and bruising calls for a change in how the crew picks and packs.

The three fungi and how they carry over

Alternaria fruit rot is caused by Alternaria tenuissima, anthracnose by Colletotrichum acutatum, and Botrytis by Botrytis cinerea. Each one survives the winter in a different place, which is why clean-up work has to target more than one part of the bush.

Alternaria tenuissima overwinters as mycelium and spores in old dried-up berries and dead peduncles, the berry stems left from the previous year. Spores produced on infected tissue spread from late bloom through fruit maturity on wind or rain splash. Leaf spots that start early in the season can later produce spores that infect berries. The infections that become postharvest rot typically enter through the stem scar after harvest. Disease development peaks around 68 degrees F, and spores produced on infected berries can spread to healthy fruit in the same storage container.

Colletotrichum acutatum survives winters in blighted twigs. Through the growing season, spores release from those twigs during rainy periods and cause latent infections on immature green fruit. The fungus sits inactive and symptomless inside the berry until the fruit begins to mature, then becomes active and rots it. Spores from diseased berries can go on to infect healthy fruit, and warm moist conditions favor the disease.

Botrytis cinerea overwinters as hard dark sclerotia or as mycelium in plant debris. Spores develop in spring on infected tissue and spread mostly on wind, sometimes on rain. Young leaves, shoots, flowers, and fruit are the most susceptible, and branch tips killed by winter injury are especially vulnerable. Wet weather between 59 and 68 degrees F drives the disease. Free water lets spores germinate and start an infection within a few hours, and spores from infected berries can infect healthy ones.

Why does pruning change the rot picture?

Inoculum is the starting balance. Pruning out and removing old and infected tissue lowers it, and opening the planting through proper spacing and pruning reduces relative humidity in the canopy. Both moves work on all three diseases at once, because all three need wet tissue and a spore supply to start.

Nitrogen is the other lever. Heavy spring nitrogen pushes rapid succulent growth, and that young tissue is more susceptible to infection. Holding back on spring nitrogen is a disease tactic, not only a vigor decision.

Which blueberry cultivars hold up

Cultivar choice can help control anthracnose. Brigitta Blue, Elliot, Hannah's Choice, Legacy, Little Giant, Morrow, Murphy, and Reveille carry some resistance to anthracnose. Berkeley, Bluecrop, Bluetta, Blueray, Bounty, Chanticleer, Coville, Harrison, Jersey, and Spartan are highly susceptible and worth avoiding where ripe rot has been a problem.

For postharvest Alternaria fruit rot, cultivars with dry stem scars such as Bluehaven help, because the stem scar is the entry point for that rot. Where Botrytis is a problem in your area, avoid cultivars with tight clusters, which hold moisture and give the fungus a place to work.

What does harvest timing have to do with rot?

Quite a lot. Harvest berries often enough that overripe fruit never sits on the bush, and keep crews out of the planting when the fruit is wet. Handle berries carefully so you do not bruise or damage them, since every wound is an opening. Prompt postharvest cooling is a major piece of fruit rot prevention, and it is the step most often squeezed when the day runs long.

Where spray programs are concerned, commercial growers are referred to Bulletin 506B2, the Midwest Commercial Small Fruit and Grape Spray Guide, and backyard growers to Bulletin 780, Controlling Diseases and Insects in Home Fruit Plantings. Both are available from a local OSU Extension office or from OSU Extension's online bookstore at extensionpubs.osu.edu. The Ohio State University fact sheet on these three blueberry fruit rots sets out the symptoms, disease cycles, and control measures in more detail, and it was first published in 2011.

Can clean seed prevent disease before it starts?

Blueberry rot control starts with what is already in the planting. Annual crops start earlier than that, at the seed. One of the ways plant pathogens enter a crop is on seeds, and bacterial pathogens are particularly good at travelling that way. The earlier a pathogen meets the crop, the greater the chance of a serious problem, which is why starting with clean seed matters so much.

Hot water treatment, properly used, kills most plant disease-causing bacteria both on and within seeds. It is suggested for eggplant, pepper, tomato, carrot, spinach, lettuce, celery, cabbage, turnip, radish, and other crucifers. Primed, old, or poor-quality seed may be damaged by the treatment, and seed coatings, including pelleting agents, pesticides, and biologicals, are partly or completely stripped, so only raw seed should be treated. Cucurbit seed such as squash, gourds, pumpkins, and watermelons may be damaged by hot water. Treat a small sample of each lot and test it for germination before committing the whole lot.

Hot water or chlorine?

Chlorine treatment is easier and needs no specialized equipment, but it kills pathogens only on the seed surface. It suits pumpkin and other large-seeded vegetables that hot water may damage and that do not commonly carry pathogens beneath the seed coat. Cucumber seed can internally harbor the bacterium behind angular leaf spot, so hot water is recommended there. Pepper seed tolerates hot water, but the main pathogens behind bacterial spot and Pseudomonas leaf spot sit on the seed surface, so chlorine is the recommendation. Chlorine also kills fungal, viral, and human pathogens on the seed coat that hot water may not reach, and the two treatments can be combined.

Hot water work lives and dies on temperature. Staying within one degree Fahrenheit of the target is critical, and that is achievable on a stovetop with constant attention, though water baths or precision water heaters make it easier. Ohio State researchers tested a sous vide style precision cooker and found it an effective, inexpensive alternative to commercial water baths.

  Seed
  Water temperature (F)
  Water temperature (C)
  Minutes




  Brussels sprouts, eggplant, spinach, cabbage, tomato
  122
  50
  25


  Broccoli, cauliflower, carrot, collard, kale, kohlrabi, rutabaga, turnip, cucumber
  122
  50
  20


  Mustard, cress, radish
  122
  50
  15


  Pepper
  125
  52
  30


  Lettuce, celery, celeriac
  118
  48
  30

The hot water sequence is fixed: pre-warm seed for 10 minutes at 100 degrees F, drain through a fine mesh sieve, transfer to water held at the temperature for that seed type, hold for the listed time while stirring occasionally, then run cold tap water over the seed for five minutes to stop the heating action. Spread the seed in a single uniform layer on a screen to dry, with a fan to move air. A sous vide setup follows the same temperatures and times, with seed in loosely packed cups or cheesecloth bags so heat reaches every seed. Chlorine treatment is shorter: agitate seed for one minute in a solution of 25 ounces of Clorox plus 100 ounces of water with one teaspoon of surfactant such as liquid detergent, using one gallon of solution per pound of seed and mixing a fresh batch each time.

Pick the treatment that fits the seed in front of you, raw and untreated, and test it on a sample first. The rest of the season depends on how clean that seed went into the ground.