Daylilies Diseases and Pests

Daylilies are usually pretty disease and pest free, if planting conditions allow for good drainage and the soil contains a good supply of organic matter.  The biggest problem I see when people tell me their daylilies aren’t “doing well” or aren’t flowering is that the daylily is planted too deep.  That crown should not be covered with more than one inch of soil.

But, just like sometimes even the toughest of us get sick, so sometimes do daylilies. 

Leaf streak (internet photo)

A common fungal disease of daylilies, especially the last couple years when we’ve had cool, wet weather, is Leaf Streak.  Leaf Streak is a fungal infection that causes yellowing along the central leaf vein, followed by brown streaking.  The yellowing usually begins at the tip of the leaf and follows the leaf downward towards the crown, leading to browning, drying out, and eventually death of the entire leaf. 

 

Prevention is the best cure, if possible.  When Mother Nature delivers the moisture, we don’t have much control over when or how it hits our daylily leaves. When we are delivering the water, we can be a bit more particular.  A drip system that delivers water directly to the soil without wetting the daylily leaves is the best way to irrigate.  If the only option is to water in a way that wets the leaves, it’s best to water in the morning when the sun and breeze will quickly dry the leaves.  When the daylily leaves are wet, whether from rain, irrigation, or dew, resist the urge to walk amongst them and admire their flowers and foliage.  The more we disturb the leaves when they are wet, the more opportunity we give fungi to set up housekeeping. 

Once the infection is evident, good housecleaning is essential.  Remove and destroy affected leaves.  These leaves do not belong in the compost pile – they should be bagged and put in the garbage.  Affected plants should be isolated from others if at all possible.  

It is difficult to “cure” a fungus once the symptoms appear, but spraying with a fungicide may slow development of the disease and prevent new growth from becoming infected with Leaf Streak.    Fungicides which have proven effective for Leaf Streak are thiophanate-methyl, mancozeb, chlorothalanil, and iprodione.   These are all marketed under various trade names, but if you look at the “active ingredient” on the label, one of these should be the top ingredient. 

The entire plant should be “doused” with the fungicide, upper and lower surfaces.  If your daylilies had Leaf Streak last year, it might be a good idea to prophylactically spray the new foliage right away in the spring, before any signs of disease.  Spraying every couple weeks during the growing season might be necessary to keep it under control.

Spring Sickness is considered a “mystery” disease – its cause is not known for sure, but it is definitely more prevalent in those areas where there are freeze / thaw cycles.  I’d never heard of this disease before the spring of 2012.  That spring, instead of my nice, bold, strappy daylily leaves, I started noticing that the leaves were growing sideways, twisting and turning every which way, sometimes ending up in a twisted mass in the center of the clump.  In addition, some of the leaves had jagged, brown edges. 

What in the world was happening?  I couldn’t see any signs of pests that might be causing the problem, no mechanical damage (e.g. no one had gotten close to them with the lawnmower or weed whacker), and no signs of fungal or bacterial infection.   So, I started doing some research and discovered “spring sickness”.  The winter of 2011 – 2012 was non-existent in North Dakota.  Most people were rejoicing in our winter with very few days below freezing and even 80 degree days in March, but we gardeners were already concerned about what this meant for our perennials.  The spring sickness in our daylilies confirmed that temperatures should just not be high enough to wear shorts in February in North Dakota. 

Spring sickness may also be caused by mites or some type of fungal infection, but in my daylilies, those not affected as severely, outgrew the problem by mid-summer and went on to bloom beautifully that year.  Those more severely deformed were just fine the spring of 2013, leading me to believe that it truly was our bizarre non-winter that was the culprit.

Bacterial soft rot is often caused by the bacteria Erwinia carotovor.  Symptoms are a soft, mushy crown and roots, often accompanied by a strong, foul odor.   Erwinia is a bacteria that is present in most soils but it doesn’t cause any problems until conditions are just right.  High temperatures, poor air circulation, poor drainage and improper fertilizing are friends of soft rot.   In these conditions, it spreads very rapidly and once soft rot becomes noticeable on the daylily, it might be too late to salvage.  If caught early enough, the daylily might be salvaged by digging it up, cutting out any affected parts, soaking the remaining part of the clump in a 10% bleach solution for 20 minutes, letting the cut surfaces dry, then re-planting in a well-drained area.

White mold is a crown rot caused by the fungus Sclerotium.  The crown of the daylily becomes covered with white, cottony looking growths.  Sometimes small, black seed-like growths are evident in the cottony growths.  There are no fungicides effective against sclerotia.  Treatment is to remove the daylily and the soil surrounding the plant for one foot beyond the root ball and one foot deep.  Both the daylily and the soil should be bagged and discarded.  Sclerotia lives in the soil for many years, so it is important not to compost any part of the plant or use the affected soil in other garden spots.
 

Rhizoctonia crown rot is another disease of daylilies that occurs in wet years, or in areas with poor drainage.  The crowns rot from the soil surface down towards and into the roots.  There is usually a dark, heavy growth on the crown of the daylily that almost looks like a piece of cloth or heavy felt wrapped around it.  Unfortunately, like the other crown rots, there is no treatment once the rot is established.  The only hope is to catch it soon enough that you can salvage some of the clump through the pruning and soaking method described for bacterial soft rot.

Pests of daylilies hardly bear talking about, in my opinion.  I have rarely, if ever, had much problem with insects.  Even slugs, those dirty little slimeballs, rarely do much damage to daylilies.  Cutworms, cucumber beetles, Japanese beetles, grasshoppers and bulb mites may cause some periodic damage, but usually not of much significance. 

Spider mites are the most common pest of daylilies, especially in hot, dry weather.  Spider mites cause yellowing of the leaves, sometimes with tiny brown spots amongst the yellow showing where the mite began sucking the clorophyll from the leaves.  The webbing is sometimes visible, especially on the undersides of the foliage, and you may even see the mites moving in the webbing.  Usually a strong spray from the garden hose or spraying with an insecticidal soap will do the trick. 

Thrips may infest daylilies and cause streaking on the leaves and buds.  They often cause deformities in the petals, leading to an unsightly flower.    There is a specific aphid that feeds only on daylilies and isn’t controlled as easily by a hosing off with plain water as most aphids, because they get deep into the folds of the foliage.  Both thrips and aphids are best controlled by a systemic insecticide if the infestation is severe.  A common systemic insecticide, dicofol, marketed as Kelthane, Hilfol, or Acarin should not be used on daylilies, as it can damage the plant.  Other systemic insecticides should be safe for use, but please keep in mind that systemic insecticides are not selective to thrips or aphids – if a butterfly or bee stops to sip the nectar from your daylilies, it will kill them, too.  In most cases, the pest problems are not severe enough to warrant these “big guns”, in my opinion.

It is really hard to find a more carefree plant than a daylily and they give so much beauty and diversity to the garden, it’s hard to go wrong with them.