By: Mary Ellen Ellis
All kinds of onions, chives, and shallots can be affected by the disease known as onion fusarium basal plate rot. Caused by a fungus that lives in the soil, the disease can be difficult to catch until the bulbs have developed and ruined by rot. The best way to manage fusarium rot is to take steps to prevent it.
What is Onion Basal Plate Rot?
Fusarium basal plate rot in onions is caused by several species of Fusarium fungi. These fungi live in the soil and survive there for a long time. The infection occurs in onions when the fungus is able to get in through wounds, insect damage, or root scars on the bottom of the bulb. Warm soil temperatures favor the infection. Temperatures in the soil between 77 and 90 degrees Fahrenheit (25 to 32 degrees Celsius) are optimal.
The symptoms of onion fusarium basal plate rot underground include rotting of the roots, white mold and soft, watery decay in the bulb that begins in the basal plate and spreads to the top of the bulb. Aboveground, the mature leaves begin to yellow and die back. Because the leaf symptoms only begin at maturity, by the time you notice the infection, the bulbs have already rotted.
Preventing and Managing Onion Fusarium Rot
Treating onion fusarium rot isn’t really possible, but good management practices can help you prevent the disease or minimize its impact on your onion yield. The fungi that cause fusarium of onion basal plates live long in the soil and tend to accumulate, so a rotation of onion crops is important.
The soil is also important and should drain well. A sandy soil in a raised bed is good for drainage.
You can minimize the chances of having fusarium rot in your onions by choosing certified disease-free transplants and varieties that have some resistance to the fungi, like Cortland, Endurance, Infinity, Frontier, Quantum, and Fusario24, among others.
When working in the garden, take care not to wound or damage the bulbs or roots underground, as wounds promote infection. Keep insects under control and provide your plants with adequate nutrients.
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The two basic types of garlic are softneck and hardneck. About 30 days after the bulbing process has begun, hardneck garlic will produce a central “flower” stalk (called a scape, Figure 1), much as onions do, except the garlic stalk is solid instead of hollow. Softneck types do not produce this stiff flower stalk and therefore have a soft neck that is more conducive to making garlic braids. Garlic grows much like onions except it has flat rather than round leaves. The bulbing process is day length dependent. It is initiated by increasing day length in April and May. Softneck bulbs are generally less winter hardy and harder to peel, with smaller cloves, milder flavor and lower yields in Michigan trials. Softneck bulbs, however, have a longer storage life than hardneck bulbs.
Figure 1. “Flower” stalk or scape produced by hardneck garlic.
VARIETY SELECTION AND CHARACTERISTICS
As mentioned earlier, the type of onion grown in South Georgia is a short-day onion that bulbs during the short days of winter (>11 hours daylength). Although limited research has been done in this area, it may be possible to grow intermediate-day onions in North Georgia however, they would not be as mild as the south Georgia Vidalia onions.
The Vidalia onion industry is controlled by a Federal marketing order that is administered by the Vidalia Onion Committee and the Georgia Department of Agriculture. This market order defines what type of onions can be grown and be marketed as a Vidalia onion. A Vidalia onion must be a yellow Granex type. These onions should be slightly flattened, broader at the distalend (top) and tapering to the proximal end (bottom) (Figure 1). In addition, rules have given the Georgia Department of Agriculture the authority to determine acceptable varieties for the Vidalia industry. Under these rules, the University of Georgia has been mandated to test all onion varieties for three (3) years before making recommendations to the Georgia Commissioner of Agriculture. Varieties that the Georgia Department of Agriculture have recommended to be grown as Vidalia onions are listed in Table 2.
Table 2. List of approved Vidalia onion varieties (2014).
|Allison||D. Palmer Seed||Mid|
|Caramelo||Nunhems USA Inc.||Mid|
|DP Sweet 1407||D. Palmer Seed||Mid|
|Georgia Boy||D. Palmer Seed||Mid|
|Gianex Yellow PRR||Seminis Seed||Mid|
|Miss Megan||D. Palmer Seed||Mid|
|Me. Buck||D. Palmer Seed||Mid|
|Nirvana||Nunhems USA Inc.||Mid|
|Nunhems NUN1002||Nunhems USA Inc.||Mid|
|Nunhems NUN1003 aka Vidora||Nunhems USA Inc.||Mid|
|Nunhems NUN1006 aka Plethora||Nunhems USA Inc.||Mid|
|Spell Sweet||D. Palmer Seed||Mid|
|Savannah Sweet||Seminis Seed||Mid|
|Candy Ann||Solar Seed||Early|
|Candy Kim||Solar Seed||Early|
|Sweet Agent||Seminis Seed||Early|
|Sweet Caroline||Nunhems USA Inc.||Mid|
|Sweet Harvest||Sakata Seed||Mid|
|Sweet Jasper||Sakata Seed||Mid|
|Sweet UNO||Enza Zaden||Mid|
|Sweet Vidalia||Nunhems USA Inc.||Mid|
Onion varieties grown in Southeast Georgia fall into three broad maturity categories early, mid-season, or late. There can however, be considerable overlap in these categories and not all varieties will perform the same as to their maturity from one year to the next.
Along with maturity, varieties will perform differently on a wide range of quality attributes, as well as yield. Varieties can differ for pungency, sugar content, disease resistance, seed stem formation, double centers, bulb shape, and bulb size. Growers should consider all of these characteristics when making decisions on variety selection. Growers wishing to try new varieties should consult University of Georgia variety trial results. Trial results should be examined over several years to get a true picture of a variety’s potential. Even after evaluating trial data, growers considering new varieties should grow them on limited acreage to get a feel for their performance potential under their growing conditions. In addition, growers wishing to grow Vidalia onions should check with the Georgia Department of Agriculture for the current allowed varieties.
Preventing and diagnosing problems and diseases:
Your best defense against plant diseases is always a healthy, vigorous plant. Good loam, good drainage, feeding and weeding will generally help your garlic fend off whatever unhelpful organisms lurk in your soil. Crop rotation and care in planting, as described above, are crucial. And yet even the most skilled gardeners are sometimes beset by problems despite their best efforts.
Garlic Rust (Puccinia allii)
Garlic Rust spreads by wind, and can easily travel from garlic to onions and leeks, or vice versa. Garlic Rust appears as small orangish bumps or pustules on the leaves. If it strikes early in the season it can kill a plant (or a whole crop), though if onset is later, the bulbs can survive and be harvested. At the first appearance of rust, cut off any infected leaves, dispose of them out of your garden (not in the compost!), and wash your hands and any tools. Even if all leaves must be removed, a stalk can continue to photosynthesize and mature a bulb. To avoid, water early in the day so that the plant can dry fully before nightfall. Make sure your crop has plenty of sun and air circulation.
Basal Rot (fusarium culmorum)
Basal Rot is slow to develop, first visible in a premature yellowing and dying back of the foliage. White fungal growth may be visible at the base of the stalk, and after harvest individual cloves or whole bulbs may rot. This is an opportunistic pathogen, so healthy plants are less vulnerable. Avoid damage to the basal plate (scab-like bottom of clove) as infection often occurs there. Most common at higher temperatures.
White Rot (Sclerotium cepivorum)
White Rot looks very similar to Basal Rot, except faster and more overwhelming in its effects. Fluffy white mold growth is visible on the stem and at the base of the plant, and small dark sclerotia become visible on the rotting bulbs. Climates with cooler summers (averaging below 75F) are most prone to White Rot. Sclerotia can survive indefinitely in soil in the absence of garlic, ready to germinate if it detects organic sulfur compounds.
Downy Mildew (Peronospora destructor)
Downy Mildew appears on the leaves rather than the stalk or base: it is also white and furry, causing yellowing of the leaves. If it strikes a young plant, Downy Mildew can be fatal in a more mature plant it will stunt growth. Harvested bulbs will appear shriveled with a blackened neck. Wet conditions are ideal for spread of these spores, though they can survive dormant for many years in dry soil.
Penicillium Decay (Penicillium hirsutum)
Penicillium Decay can infect seed cloves resulting plants will be sickly and stunted. It appears as a bluish-green mold on one or more cloves in a head. To avoid infection, discard any seed heads showing blue-green mold without opening, and also discard any with damaged basal plates. Try dusting cloves with bone meal before planting.
Stem and Bulb Nematode (Ditylenchus dipsaci)
Stem and Bulb Nematode is a tiny worm-like creature which invades the garlic stem, causing it to become swollen, spongy, and eventually rotten. Early signs of infestation include pale thick leaves on stunted plants. If nematode infection is suspected, affected plants should be carefully dug out, rather than pulled, and burned carefully. Disinfect all tools used. Avoid this organism by purchasing only disease-free planting stock from reputable growers.
Wetter climate growers may suffer from more garlic diseases. But no matter where you are located, your efforts to balance soil nutrients, rotate your crops, and take care to carefully dispose of any signs of pathogens with good sanitary practices. First and foremost, take care where you source your planting cloves! Filaree Farm of Eastern Washington (founded by Ron Engeland), Territorial Seed Co., and Hood River Garlic of Oregon all have decades of experience and solid reputations. Good stock can seem expensive at first, but the results of planting cheap infected stock can cost so much more.
Some experienced gardeners suggest caution in composting: avoid putting garlic scraps into your garden compost — or avoid using your garlic-containing compost on any beds which might grow garlic. Any garlic, from friends, farmer’s market, or supermarket, can carry disease organisms. Once well-established in your soil they will get comfortable, stay on, and ruin your future crops.
Fall-planted garlic will be in the ground longest of all your annual crops, but with a little practice it’s one of the easiest and most rewarding to grow. Set yourself up for success with these “best practices” hard-won through trial and error — others have shared their garlic lessons so you don’t have to make the same mistakes! Before long, you’ll be enjoying the rich flavors and health benefits of your home-grown garlic all year long.