Drilling Down on Disease

Integrated Approach is Best for Botrytis Blossom Blight

Botrytis blossom blight is an early-season disease impacting southern highbush blueberries. Although not frequently observed causing severe damage in Florida fields most years, infections starting during bloom can later develop into gray mold, which has become an important postharvest disease. The fungal pathogen, Botrytis cinerea, typically infects wounded or senescent floral tissues. As a blueberry bush blooms, white bell-shaped corollas (the fused petal of the flowers) should drop from the flower following pollination but before they senesce (turn brown). However, frost damage can wound plant tissues, delaying petal drop and facilitating infection of the flowers, undeveloped fruit, and damaged twigs and leaves. The pathogen survives in these dead and wounded plant tissues on the blueberry bush and on nearby weedy plants as well. Spores from these sources are abundantly produced when cool, wet periods occur during blueberry bloom through harvest. 

 

While sanitation efforts to remove diseased and infested plant materials are good horticultural practices, they won’t significantly limit the development of this disease. The development of blossom blight is highly dependent on the right environmental conditions. Botrytis has the ability to grow over a wide range of temperatures, from as low as 32°F, where growth of the fungus is very slow, to over 70°F. Moisture availability and duration are the other important environmental factors in play with the likelihood of significant disease development increasing as relative humidity and durations of leaf wetness also increase.

 

Direct loss of yield occurs when Botrytis blossom blight spreads from the corolla into the ovary and ultimately into the stem of the immature berry, sometimes killing the fruit before it can develop (Fig. 1). During periods of high relative humidity, look for a fine white-to-gray fuzz of fungal spores to be produced on infected plant parts, particularly sepals (Fig. 2). If the blight continues, an entire cluster of berries can be aborted (Fig. 3). When disease is significant, the reduction in the number of early-season berries can become economically important, and this most often occurs in years where cold snaps require extended periods of overhead irrigation for freeze protection (Fig. 4). After pollination of a flower and drop of the corolla occur, the risk of direct losses decreases; however, infection of developing fruits can continue.

 

Figure 1. Corollas of southern highbush blueberry infected with Botrytis cinerea and exhibiting typical symptoms of Botrytis blossom blight. Disease has progressed into the peduncle of the center flower.

Figure 1.  Corollas of southern highbush blueberry infected with Botrytis cinerea

Credits: P. Harmon

 

Figure 2. Gray sporulation of Botrytis cinerea is observed on corolla and calyx of southern highbush blueberry flowers after an extended period of high relative humidity. Corollas do not typically senesce and turn brown on the plant but are usually dropped after pollination while still white. Brown corollas that remain on the bush and gray sporulation are good diagnostic symptoms of Botrytis blossom blight.

Figure 2. Gray sporulation of Botrytis cinerea is observed on the corolla and calyx of flowers after an extended period of high relative humidity. Brown corollas that remain on the bush and gray sporulation are good diagnostic symptoms of Botrytis blossom blight.

Credits: P. Harmon

 

Figure 3. Severe symptoms of Botrytis blossom blight were observed on a cluster of flowers of southern highbush blueberry. The flowers that produced the immature fruits (top) developed before the cool wet period and escaped infection. Some of these fruits could be infected, but they will likely mature if no further periods of disease-favorable weather recur.

Figure 3. Severe symptoms of Botrytis blossom blight on a cluster of flowers. The flowers that produced the immature fruits (top) developed before the cool wet period and escaped infection. Some of these fruits could be infected, but they will likely mature if no further periods of disease-favorable weather occur.

Credits: P. Harmon

 

Figure 4. Many of the blighted flowers on this southern highbush blueberry will not produce fruit. When extended periods of disease favorable conditions occur during bloom, Botrytis blossom blight can reduce yield enough to impact economic return. A preventative fungicide application could have protected the blossoms during the favorable conditions and may have limited disease development.

Figure 4. Many of these blighted flowers will not produce fruit. A preventative fungicide application could have protected the blossoms during the favorable weather conditions and may have limited disease development.

Credits: P. Harmon

 

The extent that these developing, green-fruit infections reduce the crop is determined by the weather and to a certain extent, fungicide applications. During cool and wet periods, or if overhead irrigation is used, infected berries develop to be deformed and soft, often rotting on the bush at the time of ripening (Fig. 5). Not all infected fruit rot before harvest, with the pathogen lying dormant (latent) in some fruit until it is stored cool and humid after harvest. These latent infections then cause gray mold, the gray fuzzy rot commonly observed on unsold blueberries in grocery stores. 

 

Figure 5. Some blueberry fruits rot or develop shriveled and deformed if the flowers are infected by Botrytis cinerea at bloom. Diseased berries that remain on the bush produce inoculum capable of infecting other ripening fruits.

Figure 5. Some blueberry fruits rot or become shriveled and deformed if the flowers are infected by Botrytis at bloom. Diseased berries that remain on the bush produce inoculum capable of infecting other ripening fruits.

Credits: P. Harmon



DISEASE MANAGEMENT

Managing diseases caused by Botrytis is best achieved with an integrated approach.  Horticultural inputs include relying on drip irrigation except for the judicious use of overhead irrigation for freeze protection when necessary. Good weed management and pruning practices can help reduce sources of inoculum and increase air flow through the bush canopies. Hand harvesting berries can reduce introducing rotten and soft pick to the sorting and packing lines while also reducing fruit bruising and damage that can spark the development of latent infections into gray mold post-harvest. Careful transport and rapid cooling of fruit also help prevent damage and drastically slow the growth of Botrytis in harvested fruit, preventing secondary spread and prolonging the shelf life of the berries.    

 

Chemical pesticide options are available to help prevent infection and slow disease development as well. Fungicide applications prior to, during, or immediately following extended cool wet periods during bloom are the most likely to limit bloom infection and direct losses. If Botrytis becomes established at bloom, fungicide applications later in the season may be required to manage spread of the fungus to ripening berries. Applications at this time are generally less efficient and effective for disease management than those made during bloom.

 

Fungicides labeled for Botrytiscontrol on blueberries in Florida are listed in Table 1. Captan is a cost-effective fungicide that will prevent infection and Botrytis blossom blight development if applied preventatively. Some of the newer, site-specific, systemic fungicides listed in Table 1 have provided excellent control in university trials when applied preventatively, as well as soon after favorable conditions occur, but before symptoms are apparent. However, resistance to site-specific fungicides is a real concern with this pathogen. Florida blueberry isolates have been shown to be resistant to multiple fungicide groups. Resistance management strategies are included on the labels of products containing site-specific fungicides and should be followed. Each of the different fungicides in Table 1 has a different mode of action and can be rotated as part of a resistance management plan. Fungicides with low resistance risk can be used in mixtures with site specific products to help prevent management failure due to resistance. Each product's specific label instructions should always be read and followed carefully as changes may have occurred since this text was written. For additional information concerning fungicide products and other integrated approaches to blueberry production in Florida, see the 2022 Florida Blueberry Integrated Pest Management Guide (https://edis.ifas.ufl.edu/publication/HS380).

 

Table 1. 

Fungicides for control of Botrytis cinerea on blueberry.

Common name

Trade name

Activity

FRAC Group

fenhexamid1

Elevate 50 WDG

local systemic

17

cyprodinil1, fludioxonil

Switch 62.5 WG

local systemic

9, 12

boscalid1, pyraclostrobin1

Pristine

local systemic

7, 11

captan

Captan 50 WP

contact

M4

ziram

Ziram

contact

M3

1 Risk of resistance. Resistance management required for these fungicides.

 

CREDIT
DR. PHIL HARMON, Professor, Plant Pathology, UF/IFAS

& DOUG PHILLIPS, Blueberry Extension Coordinator, UF/IFAS

 
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