Diseases

Diseases

  • HVX
  • Botrytis

Hosta Virus X (HVX)

Foreword

Until a couple of years ago, this would have been a short and rather obscure, unimportant topic.  In those days hostas were healthy and carefree, with just the odd slug or snail to spoil the fun.  Shortly before the turn of the century however, there came a shocking end to this situation.

History of HVX

Somewhere around the mid 90's of the last century, an unknown virus started appearing in hostas.  A very contagious virus that often only shows its symptoms after a couple of years, if ever.  Presumably the first infected plants originated somewhere in the USA.  A number of them reached the Netherlands, the world center for industrial mass propagation and ended up in large scale tissue culture.  There were even new "cultivars", very sought after for the ink spot pattern on the leaves.
Around the year 2000, the first alarm signals came from the hosta world.

Even then it took some years before the wholesale traders began to act, and only because the export to the United States, by far the largest export market for hostas, was seriously endangered. 
The reaction of some of the grand scale commercial growers nowadays is, in my opinion, almost criminal, and shows a total lack of respect for their clients, the garden centers, landscapers and, in the end, the hosta lovers.  At the beginning of 2007, you can find contaminated plants, often in large batches all around.  It looks to me like the stocks of HVX plants are dumped on the European market. 

 I saw it happen at a local garden center, where I used to buy a couple of plants every year.  All pictures in this article were taken there on a single day, with permission of the owner.

The owner was completely unaware of the problem.  He'd been trying to offer a good assortment of named hostas, and saw all of his efforts go to waste in a few weeks time, thanks to some hosta cowboy.

I may be wrong, but it seems like only a couple of individuals are trying to stop the epidemic.  However, as long as the big players are not deeply convinced of the gravity of the situation and don't start taking the appropriate measures (testing, destruction of infected stocks, quarantine, decontamination measures) my advice is: only buy hostas from a HVX-free source.
Inform plant sellers about the phenomenon, because often they are just ignorant and appreciate the information.

How to prevent HVX ?

HVX is extremely contageous, especially because it can, contrary to earlier assumptions, survive in the soil for years.

It's of the greatest importance to stay vigilant and careful at all times.  Here are some tips to stop HVX from entering your collection:

  • only buy from reliable sources.  Voor mezelf zijn dit nog dit nog 4 gespecialiseerde kwekers-handelaars en enkele liefhebbers;
  • bestrijd slakken en vretend of zuigend ongedierte drastisch.  HVX verspreid zich via plantensap van een ge´nfecteerde plant dat in de sapstroom van een gezonde plant komt.  Dit kan zowel bovengronds, maar ook ondergronds.  Persoonlijk, jammer goenoeg door eigen ervaring, ben ik er van overtuigd geraakt dat slakken HVX kunnen verspreiden;
  • gooi nooit afval van hosta's op een composthoop als je de compost nog wil gaan gebruiken in je tuin;
  • herbruik nooit oude potgrond van hosta's, bv. als "bodemverbeteraar" in de tuin;
  • ontsmet bij de manipulatie van hostas steeds je gereedschap: reinigen, schoonmaken met bleekwater, volledig laten drogen; eventueel verhitten;
  •  een alternatief voor direct planten in vollegrond is uitplanten in een ingegraven pot of container.  Hierdoor wordt een besmetting via de grond voorkomen; indien larer zou blijken dat de uitgeplante hosta besmet was, kan je hem ook eenvoudig weer verwijderen.

Wat te doen bij HVX ?

  • als je twijfelt of een plant aangetast is, is mijn advies: gooi hem weg als het geen dure plant is.  Anders plaats je de plant ergens zo ver mogelijk weg van andere hosta's, tot je uitsluitsel hebt.  Probeer die quarantaine zo kort mogelijk te houden.
  • verwijder aangetaste planten zo snel mogelijk; doe ze in een afvalzak, breng ze naar het containerpark, maar zorg dat je niets in je tuin achterlaat;
  • bij potplanten: gooi weg met pot en al;
  • bij planten in vollegrond: verwijderd de planten zo voorzichtig mogelijk: ruim een voldoende grote kluit, zodat je GEEN wortels achterlaat, of zelfs maar kneust.  Verwijderd voldoende grond, om zoveel mogelijk van het virus te verwijderen en vervang deze door gegarandeerd virusvrije aarde;
  • plant minstens 2 jaar geen hosta's in aarde waar een besmette hosta heeft gestaan; omdat hosta's vrij ondiep wortelen is het wel mogelijk om de besmette grond volledig te verwijderen.  Wanneer je dit doet, en je bent zeker dat je alles geruimd hebt, kan je het risico nemen om terug een hosta te planten.

 

 

Botrytis

In some years a lot of spreading leaf lesions on hostas occur.

These lesions can be the result of infection with the common plant pathogen Botrytis cinerea.
In hosta collections, plants are often grown close together.  This can create a humid environment which is ideal for the development of botrytis blight.

The lesions start out as water soaked spots that increase in diameter to become circular spots with dark haloes around the outside of the lesions. As the lesions mature and increase in size, rings can be seen within the lesions that appear cinnamon to dark tan in color. The dark fungus can often be seen on the underside of the lesion especially early in the morning when the leaves are still wet. The lesions can spread down the leaf petiole and can sometimes be found even beneath the soil surface. The age of plants is not a factor in botrytis infection, but there are clearly some species and cultivars that are more vulnerable.  This does not follow a particular pattern in the plant characteristics, such as leaf texture, variegation or leaf size.

 Botrytis reproduces by spores that are spread by wind, grower activity, irrigating, spraying. Botrytis spores are released about midmorning and mid afternoon and coincide with a rapid decrease in relative humidity. If spores land on a leaf of a susceptible species of hosta but the environment is not favorable for infection, they may survive for up to three weeks.

Botrytis requires free water on the leaf surface to cause disease. It will not germinate on dry leaves and will be unable to infect plant tissue and reproduce.  Therefore preventing the accumulation of free moisture for extended periods on leaves is critical to botrytis management. Relative humidity should be kept low (below 85%) to prevent formation of condensation.  This can be achieved by good air circulation. If plants are being watered overhead, the plants should dry before evening. Containers should be spaced far enough apart to allow some air circulation so the leafs can dry.

 

Plants already affected should be isolated from tolerant varieties to reduce the amount of spore exposure to these plants. This should be done on calm days to minimize the spreading of spores. Botrytis occurs on living and dead tissue, and it is therefore important to remove leaves already diseased or senescent. Fallen leaves should be removed as that tissue is still a suitable host for infection if the environment is wet and humid. Diseased and senescent leaves should be collected and put into an enclosed trash container and disposed of, as botrytis can still produce spores on dead tissue. Hostas that have severe cases of disease, with lesions covering 50% of the leaves, or with lesions all the way down the petioles should be discarded. Don't move them around, but place a plastic bag over them to prevent the release of spores on healthy plants.

A number of fungicides is available, protective fungicides to prevent botrytis, which should be applied through the season following the time schedules provided by the producer, as well as curative ones.  These last ones are only really effective just after infection has occurred, so preventing contamination is essential.

Apply fungicides as early in the morning as possible or in the evening to allow leaves a period for drying.