MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

This year, the weather has been conducive for infection with the common plant pathogen Botrytis cinerea (view photos). In commercial nursery fields and shade houses, the practice of growing plants close together creates a humid environment, which is ideal for the development of botrytis blight. In the presence of inoculum, when conditions are favorable for development of Botrytis and the plant is susceptible, botrytis blight can occur. In hostas, immature lesions appear mostly as water-soaked spots that increase in diameter to become circular spots with dark halos around the outside. As the lesions mature and increase in size, rings can be seen within the lesions, which appear cinnamon to dark tan in color. The dark fungus often can 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 sometimes can be found beneath the soil surface. In hostas, it appears the age of plants is not a factor in Botrytis infection. There clearly are differences in susceptibility among varieties, but these do not follow any particular characteristic of the plant, such as leaf texture, variegation or leaf size.

Botrytis reproduces asexually by conidia (spores) that can be released and dispersed by plant agitation, such as wind, grower activity, irrigating, spraying pesticides and during harvest activities. In both fields and shade houses, Botrytis conidia are released in the atmosphere from mid-morning to mid-afternoon, coinciding with a rapid decrease in relative humidity. If conidia 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 and potentially infect and cause lesions even after the plants have been sold.

Like many other plant diseases, botrytis blightrequires free moisture on the leaf surface to proliferate. Therefore, preventing the accumulation of free moisture for extended periods on leaves is critical to Botrytis management. Relative humidity should, if possible, be maintained below 85 percent by promoting conditions that enhance air circulation to prevent the formation of condensation on leaf surfaces. In shade houses, if plants are being watered overhead, the irrigation should be timed to allow plants to dry before evening. In addition, containers should be spaced far enough apart to promote air circulation and allow leaf drying to occur. Botrytis conidia will not germinate on dry leaves and will be unable to infect plant tissue and reproduce.

Sanitation practices are important management techniques for the control of botrytis blight. Plants that already are affected should be isolated from tolerant varieties to reduce the amount of inoculum exposure to these plants. This should be done on calm days to avoid excessive agitation of diseased leaves. Botrytis can thrive on living and dead tissueand it is therefore important, where practical, to remove leaves that are diseased or senescent (mature). If leaves fall to the ground of the shade house or onto the potting medium, these should be removed, as this 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, as Botrytis still can produce conidia on dead tissue. Dead tissue should be disposed of at a landfill. Hostas that have severe cases of disease (lesions covering 50 percent of the leaves and with lesions all the way down the petioles) should be discarded. These plants should not be moved from their initial location, but should be placed gently into bags and sealed to prevent conidia from being released.

Several fungicides have good efficacy, but must be chosen carefully as some strains of Botrytis are resistant and can grow and reproduce in the presence of some of the fungicides that had been effective. Protectant fungicides include chlorothalonil (Daconil), mancozeb (Dithane, Penncozeb, Manzate), fenhexamid (Decree), iprodione (Chipco 26019) and some strobilurins, such as azoxystrobin (Heritage).

Ideally, fungicides should be applied as preventive controls. However, as many hosta growers may be unaccustomed to protecting their plants against Botrytis, curative strategies should include the cultural and sanitation practices described earlier. Heritage, if used, should be tank-mixed with one of the protectant fungicides, such as Daconil or Manzate, and used in alternation with a protectant fungicide every 14 days where botrytis blight has appeared. Because there are no documented cases of resistance to protectant fungicides,they should be used as the cornerstone of any crop protection strategy. Chlorothalonil products and Decree are the most effective protectants. In situations where Botrytis is a problem, full label rates should be applied at the minimum labeled frequency in enough water to attain effective cover to the foliage yet still allow some drying time. In situations where there is a history of botrytis blight, the protection program for hostas should be started approximately seven days after leaf growth resumes in the spring and continue through fall when the plants become senescent.

The following bullet points summarize the steps that should be taken to prevent the establishment of Botrytis in hostas.

Field-grown hostas

• Plant hostas as far apart as possible.
• Initiate fungicide program shortly after leaves begin to appear in the spring.
• Maintain protection program initially on a seven-day interval with thelowest use rates of the fungicide until foliage is profuse.
• When foliage is profuse, increase fungicide rate to themaximum labeled rate, and apply on a 14-day interval (note maximum permitted amount per season).
• Apply fungicides as early in the morning as possible or in the evening to allow leaves a period for drying.
• Avoid unnecessary movement of the plants, which could cause tissueinjury or damage where the fungus could develop.
• Grow plants as far away as possible from container-grown shade house hostas and other perennials.

Container-grown shade house hostas

• Place severely diseased hostas in airtight bags.
• Remove the most susceptible hostas to an isolated location on a calm day.
• Remove diseased leaves and place in anenclosed container for disposal off the farm.
• Apply fungicide immediately to plant stubs after the leaves have been removed, making sure that the fungicide saturates the soil to kill conidia that have entered the media.
• Increase spacing between plants in the shade houses.
• Water during a period of the day that allows the plants sufficient time to dry out.
• Keep botrytis blight-susceptible and botrytis blight-tolerant species in separate shade houses.
• Apply fungicides at alower rate to tolerant species and ahigher rate to susceptible species, ensuring complete coverage of the plants, but still allowing a period during the day for foliage to dry out.