# Alternaria Black Spot (_Alternaria brassicae_) Infection Severity on Cruciferous Oilseed Crops
## 4 Discussion  
Weather conditions strongly influence the Alternaria black spot development [31], and previous studies have demonstrated that under suitable weather conditions mass spread of this pathogen can occur [14, 16]. In this research, development of Alternaria black spot disease was assessed on spring oilseed rape and on five alternative cruciferous oilseed crop species. Both growing seasons, 2010 and 2011, were hotter and dryer compared to long-term (48 years) average and thus, were suitable for pathogen _A. brassicae_ development and disease assessments. Our research findings clearly showed that _Alternaria_ spp. is an important pathogen on spring oilseed rape and on some alternative oilseed crops (_B. nigra_, _B. juncea_) in Northern Baltic conditions. The optimum temperature for sporulation of _A. brassicae_ is 18-24 \({}^{\circ}\)C [32]. In our results, the main triggers for higher disease damage on some cruciferous species were warm weather conditions. Similarly, [PERSON] et al. (2020) have found in lab experiments that the infection of _B. napus_ and _B. juncea_ by _A. brassicae_ was less, 42%, under low temperatures of 14/10 \({}^{\circ}\)C (day/night) compared with higher temperatures of 22/17 \({}^{\circ}\)C (day/night), where 88.2% leaf area was infected [33]. Wet periods during the growing season are another factor promoting Alternaria black spot pathogen sporulation and disease development [32]. According to [PERSON] et al. (2011), Alternaria black spot disease incidence and severity on winter and spring oilseed rape are greater in years with a wet growing season than in years with dry or very dry growing seasons [4]. In our research, the combination of two-year weather data showed a negative relationship between temperature and relative humidity, with lower temperatures and higher humidity in both years in May and drier and warmer weather in August. Thus, the high degree of infection of _B. napus_ and _B. juncea_ by _A. brassicae_ in our study in 2010 and 2011 is consistent with the main role of growing season weather conditions as highlighted by [PERSON] et al. (2011) [4]. Figure 3: Mean area under the disease progress curve (AUDPC) of Alternaria black spot (_A. brassicae_) infection in different cruciferous oilseed crops (averages for the two study years). Error bars denote the standard error of the means. Means followed by different capital letters (A–D) above each bar indicate a significant influence (_p_ < 0.05) of cruciferous oilseed crop species (Tukey HSD test). Alternaria Brassicae is a well-known seed-borne pathogen, causing also pod infection and thereby early maturation by pod splitting and seed fall-off. In Estonia, _A. brassicae_ has been found on several species belonging to the Brassicaceae family: _Brassicae oleracea_ var. _botrytis_ L., _B. napus_ subsp. _nupus_, _B. rapa_ subsp. _oleifera_, indicating that the pathogen is able to infect a wide spectrum of cruciferous crops and weeds from the _Brassicaceae_ family and other families [27]. Plant pathogen resistance is considered the most promising control method in sustainable agriculture. Common cultivated _Brassica_ crops have very low genetic variability and are therefore highly susceptible to several diseases that can cause major yield losses [34]. In our field trial, AUDPC values were the lowest in _R. sativus_, _E. sativa_ and _S. alba_ compared to the other species. In both seasons, the infection severity was very low before the last observation week in all these three species. _Sinapis alba_ is a wild member of the Brassicaceae family and is considered to possess high resistance against Alternaria black spot [35]. On the other hand, the information on _E. sativa_ vulnerability to Alternaria black spot has been limited. According to a recent report by [PERSON] et al. (2020) [36] from Brazil, _A. brassicae_ infects older plants of _E. sativa_, and it is likely that the association between _A. brassicae_ and _E. sativa_ is widespread there, though the degree of disease damage in cultivated plants is unknown.