ALLIUMS (Onions, Garlic,and Leeks)

SCLEROTIM CEPIVORUM-also known as ALLIUM ROOT ROT 

Plant pathogen-Sclerotium Cepivorum FUNGUS

Host- alliums, Onions, garlic, leeks; Occurs all around the world where alliums are grown.  

Ideal growing conditions- The fungus thrives in cool weather and is found in the soil as small round structures called Sclerotia, These can live in the soil for decades and affect any allium planted there. 

Symptoms: One of the first things you will notice in the Allium is the foliage lacking luster.  The leaves will appear stunted and starting to yellow; eventually the older leaves will die.  In cool weather mycelium  growth will appear around the base plate and eventually covering the entire bulb, it looks white and fluffy.  The fungus can create very small hardened fungal bodies, sclerotium bodies, these will appear in the fluffy white growth.  Mycelium can grow outwards from the roots of one plant to the roots of a neighboring plant, and it is by this method that the pathogen can move down a planted row.  

Prevention: The most effective controls for white rot are avoidance and sanitation. Once a field is infected, chemical treatments are necessary to produce onion or garlic crops.  If S. Cepivorum is found in soil do not plant Alliums there for over ten years. Possible sifting methods to remove mycelium but seems extremely labor intensive.  On garlic, the disease is commonly introduced into the field on seed cloves. The best way to prevent disease from gettin onto your land is to only buy seeds from a trusted seed seller.  However, the fungus is vulnerable at temperatures above 115°F, thus dipping seed garlic in hot water will greatly reduce the amount of pathogen and is a good preventative measure, although it may not completely eradicate the fungus. Also, temperatures above 120°F may kill the garlic, so careful temperature control is essential.  

BOTRYTIS LEAF BLIGHT

Host- alliums, Onions, garlic, leeks at early bullbing stages

Pathogen- fungi Botrytis squamosa

Symptoms: First occur on oldest leaves, atiny oval white or yellow spot, Blight, Slighty sunken into the leaf and surrounded by a silver halo.  If you slice the leaf open you can find the lesion goes all the way though the leaf.  Botrytis can easily be confused with Leaf fleck, ozone injury and leaftip dieback. 

Environment-   favorable environmental conditions, high rainfall, extended periods of leaf wetness, high relative humidity, and moderately warm temperatures 50 F-75 F, can result in reduced onion bulb growth and yield.  The Squamosa fungi, compacted as sclerotic bodies,  can live in debris piles and lay dorment for a long period of time, idea conditions for spore production include wet with low winds.  Debris piles should be properly disposed of to prevent spreading. 

Prevention: proper two year crop rotation practices, and healthy seeds. To reduce the incidence and severity of botrytis leaf blight, cull piles should be destroyed, seed fields should be planned well apart from commercial onion production fields, and volunteer onions should be rogued. At harvest, severed onion tops should be removed from the field and destroyed. Fungicides are used instantly in commercial farms. 

Soft Rot:

Caused by the bacteria: Erwinia carotovora.

Symptoms:  At first the bacteria accesses the allium tissues through the bulb neck as the plant becomes mature, it becomes water logged; later becoming a decomposing slimy mess. It infects and rots certain scales or layers of the allium bulb flesh and emits a foul odor, remember> stink=bacteria.   

Environment: Alliums stored in a warm environment can become affect by E. carotovara.  Plants that have been damaged or bruised can also be affected.  These affected plants can easily spread infection by contact with other alliums.  

Management: Cultural controls. 

CARROTS

CERCOSPORA LEAF BLIGHT: 

Symptoms:  Leaf spots caused by Cercospora carotae first appear along the margins of the leaves, often causing the leaves to curl. Spots inside the leaf edges are small, roughly circular, and tan or gray to brown with a dead center (fig. 1).  As the lesions increase in number and size, the entire leaflet withers and dies. The fungus attacks younger leaves and plants in preference to older ones. In heavily infested fields, however, both older and younger leaves are subject to attack. The pathogen also produces lesions on the petioles and stems (fig. 2), characterized by dark brown borders and tan to gray centers (fig. 3). The lesions may merge and girdle the stems, causing the leaves to die. Because Cercospora leaf blight develops rapidly in hot or humid weather, it is likely to occur in July and early August in New York State.

ALTERNARIA LEAF BLIGHT:

Symptoms:  Alternaria leaf spots first appear at the margin of the leaflets and are dark brown to black and irregular in shape (figs. 4, 5, 6). Lesions produced on the petioles and stems are dark brown and often coalesce and girdle the stems (fig. 4). As the disease progresses entire leaflets may shrivel and die, appearing scorched (fig. 7). Alternaria leaf lesions are generally more prevalent on older foliage and plants than on young foliage. The disease spreads rapidly on the older leaves of a maturing crop after the rows have closed due to poor air circulation.  Root lesions are irregular, dark brown to black, and appear as a shallow, firm area of decay. Cool weather is favorable for its development as well as when temperatures are warm or when moisture in the form of rain, dew, or irrigation water is persistent, the threat of infection and rapid spread of leaf blight organisms is high.

Control:  A two-to three-year rotation is recommended to allow for natural decline in the pathogen population. Fall plowing or disking is also recommended to hasten the decomposition of plant residues and to reduce the number of overwintering bacteria and fungal spores.  The use of disease-free seed is strongly recommended because all three pathogens can survive on or in the seed. Hot water treatment of seeds reduces bacterial leaf blight caused by Xanthomonas carotae.  Because Alternaria leaf blight destroys the mature foliage, it is important to encourage the continuous production of young vigorous leaves for gripping by a mechanical harvester. Research has shown that well-fertilized carrots are less likely to develop Alternaria leaf blight. A nitrogen application made in mid-August or early September may be beneficial in promoting foliage development. An alternative method of applying nitrogen is to add sprayable urea to each fungicide spray. Although only a small amount of the foliar-applied nitrogen is absorbed, the excess is washed to the ground and taken up by the roots.

CARROT POWDERY MILDEW:

Symptoms:  Fluffy white growth on stems and leaves.  Foliage becomes brittle, and may eventually shrivel and die.  Severe infection can result in foilage loss, making harvesting difficult and lowering crop yields.

Control:  Infection is favored by high humidity and moderate temperatures.  Carrots are more susceptible 7 weeks after planting.  Water stress will increase disease severity.  Preventative control measures include:  monitor young crops regularly, limit the movement of machinery and equipment from infected area, maintain good even crop growth with adequate fertilizer and irrigation program.

Treatment:  Sulfur is commonly used to control powdery mildew but other fungicides such as strobilurins are also effective. Some cultivars may be more tolerant than others.

ROOT DIEBACK (FORKING AND STUBBING):

Symptoms:  Root dieback of carrots produces excessively branched or stubbed roots. The fungus kills young tap roots less than 2 weeks after seed germination, reducing root length and/or stimulating multiple root formation (forking).  Can also cause damping-off.

Control:  Avoid overwatering and provide good drainage. Rotating to small grains might reduce soil populations of some Pythium spp.